The Gem Lexicon

Aquamarine

Aquamarine is the blue to greenish-blue variety of beryl coloured by iron, typically far cleaner than emerald and commonly heated to remove greenish and yellowish secondary tones.

Most aquamarine reaching the market is heated, a permanent and accepted treatment that should still be disclosed. Because the rough is often clean and large, price is driven mainly by depth of colour rather than clarity, and pale material is far more common than richly saturated blue. Small stones rarely show strong colour, so a convincing blue usually means a larger stone.

See also: Beryl · Morganite · Heat treatment

Beryl

Beryl is the beryllium aluminium silicate species behind emerald, aquamarine and morganite, with a Mohs hardness of 7.5 to 8 and colours that depend almost entirely on trace elements.

Chromium or vanadium produces emerald, iron produces aquamarine, manganese produces morganite, and colourless beryl is called goshenite. Hardness is consistent across the family but toughness is not: clean aquamarine and morganite behave well in rings, while heavily included emerald is brittle and needs a more protective setting. Beryl also has one direction of imperfect basal cleavage, which cutters and setters work around.

See also: Emerald · Aquamarine · Morganite

Chrysoberyl

Chrysoberyl is a beryllium aluminium oxide species of Mohs 8.5 — harder than spinel or topaz — whose varieties include alexandrite and the cat's-eye stone known as cymophane.

Despite the name, chrysoberyl is a separate species from beryl, although both contain beryllium. Ordinary yellow-green to golden chrysoberyl is durable, affordable and under-appreciated, while the alexandrite and cat's-eye varieties sit at the top of the market. Cat's-eye chrysoberyl owes its sharp band of light to parallel fibre-like inclusions, and is the only gem the trade may call simply "cat's eye" without further qualification.

See also: Alexandrite · Chatoyancy · Spinel

Citrine

Citrine is the yellow to orange-brown variety of quartz; genuinely natural citrine is uncommon, and most stones sold under the name are amethyst heated until the purple turns golden.

The conversion is permanent and universally accepted in the trade, but it should be disclosed, and the price gap between heated and unheated citrine is modest because both are plentiful. Heated material often carries a reddish or orange cast and is sold as Madeira citrine, while unheated stones lean towards a lighter lemon; separating them reliably calls for laboratory work. Ametrine, the natural purple-and-yellow bicolour from Bolivia, is a related curiosity.

See also: Quartz · Amethyst · Heat treatment

Corundum

Corundum is the mineral species — crystalline aluminium oxide — that yields both ruby and sapphire, ranking 9 on the Mohs scale and second only to diamond in hardness among gem materials.

Because corundum is defined by chemistry rather than colour, sapphire and ruby differ only in trace elements: chromium gives ruby its red, while iron and titanium produce the classic blue. This matters when comparing prices, since one species carries very different values depending on colour, clarity and treatment. Hardness is not the same as toughness, so a sharp knock can still chip a facet edge or an exposed girdle.

See also: Sapphire · Ruby · Mohs scale

Demantoid

Demantoid is the green variety of andradite garnet, distinguished by dispersion higher than diamond's and, in Russian stones, by the horsetail bundles of chrysotile fibres treated as a hallmark.

At Mohs 6.5 to 7, demantoid is the softest of the mainstream garnets, so it is better suited to earrings and pendants than to a ring worn every day. Russian Ural stones command a premium, partly for colour and partly for the horsetail, one of the few inclusions that raises rather than lowers value. Namibian and Madagascan demantoid lacks horsetails and often runs more yellowish-green.

See also: Garnet · Tsavorite · Fire (dispersion)

Emerald

Emerald is the green to bluish-green variety of beryl coloured by chromium or vanadium, judged first on colour rather than clarity because inclusions are expected in very nearly every stone.

The internal garden of inclusions, known as the jardin, is part of the stone's identity, but it also makes emerald brittle despite a hardness of 7.5 to 8. Nearly all emeralds are oiled or resin-filled to reduce the visibility of surface-reaching fractures, graded none, minor, moderate or significant, and that grade materially affects price. Never clean an emerald in an ultrasonic or steam cleaner, and expect filling to need renewing over decades.

Garnet

Garnet is a group of related silicate species — including almandine, pyrope, spessartine, grossular and andradite — that spans Mohs 6.5 to 7.5 and nearly every colour, blue being by far the rarest.

Garnets are among the very few gems that reach the market essentially untreated, which suits buyers wary of disclosure questions. The species blend chemically into one another, so many trade names — rhodolite, malaia, mandarin — describe intermediate compositions rather than distinct minerals. Values range enormously: common red almandine is inexpensive, while fine tsavorite and demantoid compete with sapphire. The softer members deserve protective settings in rings worn daily.

See also: Tsavorite · Demantoid · Rhodolite

Jadeite

Jadeite is one of the two minerals sold as jade, the rarer and more valuable of the pair, a pyroxene of Mohs 6.5 to 7 whose finest translucent green material is known as imperial jade.

Nephrite, an amphibole, is the other jade; the two were distinguished only in the nineteenth century, and both are exceptionally tough because of their interlocking structure. The trade grades jadeite in types: Type A is untreated, Type B has been bleached and polymer-impregnated, and Type C is dyed. Type B and Type C material can look convincing when new and deteriorates over years, so a laboratory report is essential on anything of significance.

See also: Toughness · Treatment disclosure · Dyeing

Mohs scale

The Mohs scale ranks a mineral's resistance to scratching from 1 (talc) to 10 (diamond), placing corundum at 9, topaz at 8, beryl at 7.5 to 8 and quartz at 7.

The scale is ordinal rather than proportional: the gap between diamond at 10 and corundum at 9 is far wider than the gap between 8 and 9. Hardness describes scratching only, so a hard stone can still chip or break — emerald is 7.5 to 8 yet brittle in practice. For a ring worn every day, look for 8 and above, and consider toughness alongside the Mohs number.

See also: Toughness · Corundum · Cleavage

Morganite

Morganite names the pink to peachy-orange variety of beryl, coloured by manganese and routinely heated to drive off yellow tones and leave a purer pink.

Morganite is usually a delicate colour, so cutters favour larger sizes and deeper proportions to build up visible pink; the same material can look almost colourless in a small calibrated stone. Heating is standard and permanent. A minority of material is reported to lose colour under prolonged strong light or heat, so assess a stone in indirect daylight and keep it out of direct sun.

See also: Beryl · Aquamarine · Heat treatment

Opal

Opal is a hydrated silica holding roughly 3 to 10 per cent water, valued for the play-of-colour created as ordered silica spheres diffract light, and graded on a body-tone scale running N1 black to N9 white.

At Mohs 5.5 to 6.5, opal is soft enough to need a protective setting and gentle handling. Black opal from Lightning Ridge, at the dark end of the body-tone scale, is the most valuable type, and solid Australian opal is stable in normal wear. Doublets and triplets are assembled from thin slices, are worth far less than solid stone, and can separate if soaked. Most Ethiopian opal is hydrophane, absorbing water and turning temporarily transparent or cloudy.

See also: Body tone · Play-of-colour · Doublet

→ Opals in stock

→ The Opal Buyer’s Handbook

Paraiba tourmaline

Paraiba tourmaline is copper-bearing tourmaline whose neon blue-to-green glow comes from copper and manganese; the name now functions as a variety description covering Brazilian, Nigerian and Mozambican material.

Laboratories confirm copper by chemical analysis, and a report is essential because origin — Brazil against Mozambique or Nigeria — can multiply the price of otherwise similar stones. Most material is heated to reduce the reddish component and lift the blue, a permanent change that should be disclosed. Brazilian rough is typically small, so clean stones above a carat are scarce, while the African deposits supply larger sizes.

Quartz

Quartz is silicon dioxide and the most abundant gem mineral at Mohs 7, supplying amethyst, citrine, rose and smoky quartz as well as the chalcedony family of agate, onyx and carnelian.

Because quartz is common, price at ordinary sizes reflects colour, cutting and weight rather than rarity, and large clean stones remain affordable. Airborne dust is largely quartz, which is why a hardness of 7 is the practical dividing line for everyday wear: anything softer will gradually dull with handling. Synthetic quartz is grown industrially in large volume, and synthetic amethyst can be difficult to separate without laboratory testing.

See also: Amethyst · Citrine · Synthetic

Rhodolite

Rhodolite is the trade name for purplish-red to raspberry-pink garnet of intermediate pyrope-almandine composition, usually clean, untreated, and more affordable than the ruby or pink sapphire it resembles.

Sitting between pyrope and almandine in composition lifts rhodolite out of the brownish red of common garnet and into a clearer purplish tone. It is normally untreated and often eye-clean at sizes where sapphire would be costly, with material coming from Tanzania, Mozambique, Sri Lanka and India. The main caution is tone: darker almandine is sometimes sold as rhodolite, and over-dark stones lose life, so compare candidates side by side in indirect daylight.

Ruby

Ruby is red corundum coloured by chromium; where the colour falls short of red into pink or purplish tones, laboratories generally call the stone pink sapphire instead.

The boundary between ruby and pink sapphire is a judgement call and laboratories draw it differently, which can move a stone's price considerably. "Pigeon's blood" is a laboratory colour opinion rather than a species, an origin or a guarantee of quality, and the major labs apply it to slightly different ranges. Buyers should also separate genuine ruby from lead-glass-filled composite material, which is fragile and worth a fraction as much.

→ Rubies in stock

→ The Ruby Buyer’s Handbook

Sapphire

Sapphire is gem corundum in any colour other than red, most familiar in blue but also occurring in pink, yellow, green, violet, colourless and the pinkish-orange padparadscha.

A hardness of 9 and the absence of cleavage make sapphire one of the most practical stones for a ring worn every day. Most sapphire on the market has been heat-treated to improve colour and clarity; heat is permanent and accepted by the trade, but it must be disclosed, and unheated stones of fine colour command a premium. Ministry of Gems discloses treatment on every stone.

See also: Corundum · Ruby · Padparadscha

→ Loose gemstones & sapphires

→ The Sapphire Buyer’s Handbook

Spinel

Spinel is a magnesium aluminium oxide species of Mohs 8, occurring in red, hot pink, grey, lavender and cobalt blue, and historically mistaken for ruby in several royal collections.

Most spinel reaches the market untreated, which is unusual among coloured stones and part of its appeal. Fine red and hot-pink material from Myanmar and Tanzania, and cobalt-blue spinel from Vietnam and Sri Lanka, have seen sharply rising trade demand. Flame-fusion synthetic spinel has been made for a century and is common in older jewellery, so a laboratory report is worth having on any significant stone.

Topaz

Topaz is an aluminium fluorosilicate of Mohs 8 carrying one direction of basal cleavage so complete that a misplaced knock can split a stone; most blue topaz is irradiated colourless material.

Blue topaz is abundant and inexpensive because irradiation and heating produce it readily from colourless rough, and irradiated gems must be cleared for residual radioactivity before sale. Avoid ultrasonic cleaners and choose settings that shield the girdle, since cleavage takes no account of how hard the stone is. Natural pinkish-orange to sherry imperial topaz, chiefly from Ouro Preto in Brazil, is scarce and priced in a different bracket.

Tourmaline

Tourmaline is a boron silicate group rather than a single mineral species, Mohs 7 to 7.5, spanning a wider colour range than any other gem and including bicoloured watermelon crystals.

Trade names describe colours within the group: rubellite for pink to red, indicolite for blue, verdelite for green. Tourmaline is strongly pleochroic, so a cutter's orientation affects the face-up colour as much as the rough itself does. Heating and irradiation are common and should be disclosed. Heavily included rubellite is more fragile than its hardness suggests and is better set with some protection.

See also: Paraiba tourmaline · Pleochroism

Tsavorite

Tsavorite is the vivid green, chromium- and vanadium-coloured variety of grossular garnet from Kenya and Tanzania, reaching emerald-like colour without emerald's oiling or fracture-filling.

Green grossular was first found in north-east Tanzania in 1967 and near Kenya's Tsavo National Park in 1971, and Tiffany & Co. introduced the name tsavorite in 1974. It is effectively always untreated and usually cleaner than emerald of comparable colour, and its high refractive index gives noticeable brilliance. The constraint is size: the rough is small and fractured, so clean stones above two or three carats are scarce and prices climb steeply with weight. At Mohs 7 to 7.5 it suits rings, though not rough daily wear.

See also: Garnet · Emerald · Demantoid

Turquoise

Turquoise is a hydrated copper aluminium phosphate valued for its blue to green colour, soft at Mohs 5 to 6 and porous enough that most material reaching the market has been stabilised.

Colour runs from sky blue, where copper dominates, towards green as iron substitutes for aluminium in the structure, and the dark veining running through many stones is host rock, known as matrix. Untreated turquoise absorbs oils, cosmetics and perspiration and gradually darkens or greens, which is why stabilisation with resin is standard practice and should be disclosed. Waxing, dyeing and reconstituted material are also common, and imitation in dyed howlite or plastic is widespread.

See also: Stabilisation · Dyeing · Simulant

Zircon

Zircon is a natural zirconium silicate with high refractive index, strong dispersion and marked double refraction — a completely different material from cubic zirconia, the synthetic diamond simulant.

Blue zircon, the most commercially familiar colour, is produced by heating brown rough from Cambodia and is stable and disclosable. Hardness runs from about 6 to 7.5 depending on the type, and facet edges abrade more readily than corundum's, so ring stones show wear over years. Strong double refraction makes back facets appear doubled, a useful identification clue and, in larger stones, a slight softening of the reflections.

See also: Cubic zirconia · Birefringence · Heat treatment

Sapphire Terms

The vocabulary specific to sapphire — our own field, and the one most often misused.

Australian sapphire

Australian sapphire is basalt-hosted corundum mined chiefly in the New England field of New South Wales and the Central Queensland gemfields, yielding deep blues, greens, golden yellows and parti-coloured stones.

Australia was once among the largest sapphire producers in the world, though much of the rough left the country to be cut and sold under other names. Higher iron gives Australian blue its depth and, in poorly oriented stones, an inky or greenish cast, so cutting matters more here than with Ceylon material. Parti and teal stones from these fields are frequently unheated, and traceable Australian origin now carries a premium with buyers who care about provenance.

Basalt-hosted sapphire

Basalt-hosted sapphire is corundum carried to the surface by alkali basalt volcanism, typically iron-rich and inert under ultraviolet light, and characteristically deep blue, green, yellow or parti-coloured.

Australia, Thailand, Cambodia, Nigeria and parts of Madagascar and China produce this type, and the high iron content that deepens the blue also suppresses fluorescence, which laboratories use as an origin clue. These stones are often darker and can look inky in weak light, so cutting angles and stone depth matter more than usual. Parti and teal colours are largely a basaltic phenomenon, and much of this material reaches the market unheated.

Ceylon sapphire

Ceylon sapphire is corundum from Sri Lanka, Ceylon being the island's former name, typically metamorphic in origin, low in iron, and known for bright medium-toned blues and a wide range of fancy colours.

Sri Lanka has produced sapphire for more than two thousand years and remains the broadest single source, yielding blue, pink, yellow, white and padparadscha from the Ratnapura and Elahera fields. Low iron content gives Ceylon blues their lighter, livelier look compared with basaltic stones. The word is also used loosely as a colour description, so ask whether it means where the stone was mined or only how it appears; Ministry of Gems mines its own ground in these fields.

See also: Metamorphic sapphire · Geuda · Padparadscha

→ Sapphire origin guide

Colour-change sapphire

Colour-change sapphire is corundum that shifts hue with the light source, typically blue or blue-green in daylight and violet to purplish-red under incandescent light, an effect caused chiefly by vanadium.

Value follows the completeness and drama of the change: a stone that shifts fully and cleanly is worth considerably more than one that merely darkens. Sri Lanka, Madagascar and the Umba valley in Tanzania are the main sources. Modern LED and fluorescent lighting can mute the effect, so ask to see the stone under both indirect daylight and a genuinely warm incandescent light before deciding.

→ Loose gemstones & sapphires

Cornflower blue

Cornflower blue is a trade description for a medium-toned, slightly violetish blue sapphire of soft, velvety appearance, historically associated with Kashmir material and applied subjectively across the market.

No universal standard defines cornflower blue; it is a comparison to a flower, and sellers apply it generously. Genuine Kashmir stones are scarce and trade at the highest levels of the coloured-stone market, so the phrase attached to an ordinary sapphire adds words rather than value. When a laboratory does use a colour term such as cornflower or royal blue, treat it as that laboratory's opinion under its own lighting conditions, not a species or origin claim.

→ Blue sapphires in stock

Fancy sapphire

Fancy sapphire covers every colour of gem corundum except blue, including pink, yellow, orange, purple, green and colourless stones, with red excluded because red corundum is called ruby.

The word fancy describes the colour category, not the quality: a fine yellow or pink sapphire can outrank a mediocre blue. Fancy colours are usually named as a colour plus sapphire, such as green sapphire or purple sapphire, while pinkish-orange stones may qualify for the separate padparadscha designation. Buyers should still ask about treatment, since several fancy colours, yellow and orange in particular, are commonly produced or improved by heat or diffusion.

See also: Padparadscha · Yellow sapphire · Pink sapphire

Geuda

Geuda is the Sri Lankan trade name for milky, semi-translucent white or greyish corundum that looks near-worthless in the rough but can turn fine blue when heated to high temperature.

Sri Lankan and Thai burners proved in the 1970s that geuda could be transformed, and the discovery changed world sapphire supply by converting stockpiled material into commercial blue. The colour arises as fine rutile silk dissolves and titanium and iron reorder within the crystal; the change is permanent, and stones treated this way are heated sapphires that must be disclosed as such. Not all geuda responds, and burners still work partly by experience.

See also: Milky sapphire · Silk · Unheated sapphire

Metamorphic sapphire

Metamorphic sapphire forms in rocks recrystallised by heat and pressure, such as marble and gneiss; low in iron, these stones tend towards brighter, lighter blues and often fluoresce under ultraviolet light.

Sri Lanka, Myanmar, Kashmir and the Ilakaka field in Madagascar are the classic metamorphic sources, and most stones are recovered from alluvial gravels rather than from hard rock. Low iron allows the faint red fluorescence that laboratories use as an origin clue, and it is part of why Ceylon and Burmese blues look livelier than basaltic stones of similar depth of colour. Metamorphic and basaltic are the two broad geological families a gemmologist weighs first when assessing origin.

Milky sapphire

Milky sapphire describes corundum whose transparency is clouded by dense microscopic inclusions or fine silk, giving a hazy, sleepy appearance that dulls brilliance even when the body colour is good.

Milkiness is a clarity fault, and it reduces value more than a single visible inclusion of the same size would, because it affects the whole stone rather than one spot. Light haze is sometimes called sleepy in the trade, and in Kashmir sapphire a fine velvety texture from microscopic inclusions is prized rather than penalised, a distinction of degree that takes experience to judge. Heat treatment is often used to clear milkiness by dissolving silk.

See also: Geuda · Silk · Star sapphire

Padparadscha

Padparadscha is a colour category of sapphire rather than a separate species, describing corundum that blends pink and orange in a single stone, named from the Sinhalese word for lotus blossom.

Laboratories differ on where padparadscha begins and ends, so two reports on the same stone can disagree; the name is a colour opinion, not a guarantee. Classic material comes from Sri Lanka, and demand has pushed prices well above ordinary pink or orange sapphire. Because beryllium diffusion can produce a similar appearance, a report from an independent laboratory covering both colour and treatment is worth its cost.

Parti sapphire

A parti sapphire is a single corundum crystal showing two or more distinct colours, most often blue, green and yellow, divided into visible zones rather than blended together.

Parti stones are a signature of the Australian basalt-hosted fields in New South Wales and Queensland, and were once discarded as unsaleable before designers took them up. Value depends on how cleanly the colours meet and how the cutter has oriented the zoning to show face-up. Each stone is unique, which makes matching a pair difficult and usually dearer per carat.

→ Teal & parti sapphires in stock

Pink sapphire

Pink sapphire is corundum coloured by traces of chromium, sitting on a continuum with ruby, and where the red becomes strong enough to be called ruby is a judgement laboratories make and do not always share.

That boundary has real commercial consequences, since the same stone described as ruby may be worth considerably more than as pink sapphire, and different laboratories draw the line differently. Sri Lanka and Madagascar supply most fine pinks, and hot pink material is often heated to clear silk and lift colour. Natural chromium pink is a stable colour and does not fade with light exposure, unlike some irradiated yellow and orange sapphires.

See also: Fancy sapphire · Padparadscha · Ruby

→ Pink sapphires in stock

Royal blue

Royal blue is a laboratory and trade colour description for sapphire of deep, saturated blue in medium to dark tone; like all such terms it is an opinion, not a measurement.

Some laboratories, including GRS and Gübelin, issue royal blue as a formal colour call, while others decline to grade colour by name at all, so the same stone may or may not earn the phrase. Royal blue is most often associated with Burmese and Sri Lankan material, and a stone that tips too dark loses brightness and value. Tone is where buyers are most often disappointed, so view the stone yourself in indirect daylight.

See also: Cornflower blue · Metamorphic sapphire · Zoning

→ Blue sapphires in stock

Silk

Silk is the gemmological term for fine, needle-like rutile inclusions arranged along a corundum crystal's growth directions, which scatter light, can create asterism, and largely dissolve when a stone is heated.

Because heat destroys or alters silk, intact needles seen under magnification are one of the strongest indications that a sapphire has never been heated, and gemmologists examine them closely for that reason. In moderation silk softens the light and can give a velvety glow; in quantity it produces milkiness and dulls the stone. Silk concentrated in three directions and cut as a cabochon yields a star.

Star sapphire

A star sapphire is a cabochon-cut corundum displaying asterism, a six-rayed star of light produced by reflection from three intersecting sets of needle-like rutile inclusions aligned with the crystal's structure.

The star should be centred, its legs straight and mobile under a single light source; a lopsided or weak star sharply reduces value, as does an unattractive body colour. Twelve-rayed stars occur rarely, where two generations of inclusions are present. Note that diffusion-treated star sapphires and synthetic Linde stars circulate widely, the latter recognisable by their very even colour and unnaturally regular rays.

See also: Silk · Milky sapphire · Cabochon

→ Loose gemstones & sapphires

Teal sapphire

Teal sapphire is a modern trade description for corundum sitting between blue and green with neither colour clearly dominant; it is a market term, not a recognised laboratory colour grade.

The term rose with the bridal market after 2015 and now covers a wide band of stones, from a blue that merely leans green to a genuine even split. Because no laboratory certifies teal, two sellers can apply it to visibly different colours, so judge the stone in indirect daylight rather than by the description. Much teal material comes from Australia and Madagascar, and a good deal of it is unheated.

→ Teal sapphires in stock

Unheated sapphire

An unheated sapphire has never been subjected to thermal treatment, its colour and clarity being entirely as the earth produced them, and that status can only be confirmed by laboratory examination of its inclusions.

Most sapphire and ruby on the market is heated, a permanent and long-accepted process that must nonetheless be disclosed, so unheated stones of good colour are the minority and are priced accordingly. Intact rutile silk, undamaged crystal inclusions and unaltered fluid features are what a gemmologist looks for. For any significant purchase, ask for a report from an independent laboratory rather than relying on a seller's word; Ministry of Gems discloses treatment on every stone.

See also: Silk · Geuda · Heat treatment

→ Unheated sapphires in stock

White sapphire

White sapphire is colourless corundum, free of the trace elements that colour other sapphires; at Mohs 9 it wears well in daily jewellery but shows far less fire than diamond.

Its lower refractive index and dispersion give a softer, glassier look than diamond, and stones can appear greyish as they collect surface oils, so white sapphire benefits from regular cleaning. Truly colourless natural material is less common than most buyers expect; many stones are faintly grey or yellow, and some are heated pale-coloured sapphire. It remains a durable and comparatively affordable centre stone for engagement rings.

See also: Fancy sapphire · Milky sapphire · Mohs scale

→ Yellow & white sapphires in stock

Yellow sapphire

Yellow sapphire is corundum coloured by trace iron, ranging from pale lemon through canary to deep golden honey, and it is among the most commonly heat-treated and diffusion-treated colours on the market.

Sri Lanka is the classic source, and untreated yellows of strong saturation are far scarcer than the volume of yellow sapphire in the market suggests. Beryllium diffusion can create vivid golden and orange tones, and because it penetrates the whole stone it cannot be polished away, which is why a treatment report matters. Yellow sapphire also carries astrological significance in South Asia, where demand concentrates on unheated stones of stated origin.

→ Yellow & white sapphires in stock

Zoning

Zoning is the uneven distribution of colour or growth structure within a gemstone, seen as straight bands, angular layers or patches that record changes in the crystal's growth conditions.

Colour zoning has direct commercial consequences: a sapphire with a narrow blue band must be oriented so the colour reads across the face-up view, which constrains the cutter and often costs weight. Straight, angular zoning follows the crystal's flat growth faces and points to natural origin, whereas curved growth lines indicate flame-fusion synthetic corundum. Strong zoning is common in both Ceylon and Australian sapphire, and Ministry of Gems orients zoned rough for face-up colour before cutting and checks the finished stone in indirect daylight.

See also: Parti sapphire · Silk · Synthetic

Treatments & Enhancement

What can be done to a stone, what it is worth afterwards, and what must be disclosed.

Beryllium diffusion

Beryllium diffusion is a high-temperature corundum treatment in which beryllium atoms migrate throughout the stone, altering its colour permanently and to full depth rather than forming a removable surface layer.

Because beryllium atoms are small enough to travel through the whole crystal, the colour cannot be removed by recutting or repolishing, a point often misunderstood by buyers. The treatment produces many of the orange, yellow and pinkish-orange sapphires on the market, some of which resemble padparadscha. Detection requires chemical analysis such as LA-ICP-MS, so a report from a well-equipped laboratory is the only reliable safeguard.

Cedarwood oil

Cedarwood oil is the traditional natural filler for emeralds, chosen because its refractive index lies close to that of beryl, which makes treated fissures far less visible to the eye.

Its refractive index of about 1.51 sits close to emerald's, which is why filled fissures largely disappear from view. The trade regards cedarwood oil as the most conservative filler because it is reversible: a stone can be cleaned and re-oiled without permanent alteration. Over years the oil may dry or discolour slightly, and heat from repair work will drive it out, so emeralds should be unset before any torch work.

See also: Oiling · Resin filling · Emerald

Clarity enhancement

Clarity enhancement is the umbrella term for treatments that improve a gem's apparent clarity, including the oiling and resin filling of emerald, the lead-glass filling of ruby, and the laser drilling of diamond.

Filling masks a fracture without healing it: the break remains and the stone is still weaker along it, while laser drilling opens a permanent channel so that acid can bleach a dark inclusion. Durability depends on the method and the filler, and the one phrase covers both a lightly oiled emerald and a lead-glass-filled ruby, which is classed as a composite rather than a treated stone. Keep filled stones out of ultrasonic and steam cleaners, and pass the disclosure on at every resale.

Coating

Coating applies a thin film of colour, metal oxide or polymer to a finished gemstone's surface, changing its appearance while the material underneath remains entirely unaltered.

Coated stones include the iridescent topaz sold as mystic, coated tanzanite, coated quartz and some coated diamonds. The film is only microns thick, so it scratches, wears at the girdle and can be stripped by ultrasonic cleaning, abrasive polishes or a jeweller's repolishing wheel. Because the treatment is superficial and impermanent, coated material sits at the low end of the market and must always be disclosed.

Cubic zirconia

Cubic zirconia is a manufactured zirconium oxide with no natural counterpart, used chiefly as a diamond simulant; it is Mohs 8 to 8.5, markedly denser than diamond and considerably more dispersive.

Cubic zirconia is a simulant rather than a synthetic: it imitates the look of diamond without sharing its chemistry or crystal structure, unlike laboratory-grown diamond, which is chemically identical to natural. It can usually be separated in seconds, being noticeably heavy for its size, showing exaggerated rainbow fire, and softening at the facet edges with wear where diamond stays sharp. It is also produced in colours that stand in for sapphire and ruby, so unusually clean, perfectly matched coloured stones offered cheaply are worth testing.

See also: Simulant · Synthetic · Fire (dispersion)

Doublet

A doublet is an assembled stone of two bonded layers, typically a thin slice of genuine gem material cemented to a backing of cheaper material, sold as a lower-priced alternative to a solid stone.

Opal doublets bond a slice of precious opal to a dark backing of potch or ironstone, which darkens the background so the colour reads like black opal at a fraction of the price; a triplet adds a clear quartz or glass cap on top. Both are legitimate products when disclosed, but they are worth far less than solid opal. Moisture attacks the glue line, so they should never be soaked or cleaned ultrasonically. Viewed edge-on, a straight join line gives an assembled stone away.

Dyeing

Dyeing introduces colouring agents into a porous or fractured gem so that it appears more strongly coloured, a treatment long used on chalcedony, lapis lazuli, jadeite, pearls and low-grade corundum.

Under magnification the dye usually gives itself away, pooling in fractures and along grain boundaries rather than sitting evenly through the material. Many dyes fade in sunlight or lift with solvents, acetone and perspiration, so the colour is not considered permanent. Dyed jadeite is often bleached and polymer-impregnated as well, a combination that carries a fraction of the value of untreated jade.

See also: Stabilisation · Jadeite · Treatment disclosure

Flux healing

Flux healing is a heating process in which borax or a similar flux is applied to corundum, entering surface-reaching fissures and promoting partial recrystallisation that leaves glassy residue inside the healed break.

Much of the ruby now heated in Thailand and Sri Lanka is treated this way, and reports describe it as heated with residues in fissures, graded minor, moderate or significant. It is a matter of degree: modest residue from a genuine heating process sits far above a lead-glass composite, whose apparent clarity comes mostly from glass. Significant residue still lowers value and leaves the same acid and heat sensitivities.

Fracture filling

Fracture filling covers any process that introduces a foreign substance, whether oil, wax, resin or glass, into a stone's surface-reaching breaks so they reflect less light and become less noticeable.

The word covers treatments with completely different consequences, from an oiled emerald that the trade accepts to a lead-glass ruby that is barely a gemstone at all. Filling never repairs the break; it only makes it harder to see, so the stone remains structurally weaker along the fracture. Ask what substance was used, how much, and whether the filling can be damaged by heat, acid or ultrasonic cleaning.

Heat treatment

Heat treatment subjects a gemstone, most often corundum, to controlled high temperature in order to improve colour, dissolve silk and increase transparency, producing a permanent change that must be disclosed.

Most sapphires and rubies in the market have been heated, and the trade accepts the practice provided it is declared. Heat is permanent and stable: it does not fade, and ordinary wear, cleaning or setting work will not reverse it. Because unheated stones of comparable quality are scarcer, they trade at a premium, so heating status is one of the largest single influences on price.

See also: Unheated · Flux healing · Treatment disclosure

Irradiation

Irradiation exposes a gemstone to electrons, gamma rays or neutrons in order to alter its colour centres, giving a change of colour that may be permanent or may fade with light and heat.

Nearly all blue topaz is irradiated and then heated; the colour is stable, and treated material is held until residual radioactivity falls within regulated limits before it may be sold. Other results are less durable, and irradiated yellow or orange sapphire can fade with exposure to light or heat. Irradiation is also used on diamond, pearl and quartz, and is never detectable by eye alone.

See also: Topaz · Heat treatment · Treatment disclosure

Lattice diffusion

Lattice diffusion is the technical term for heating a gemstone together with chemical elements that enter its crystal lattice and change its colour, either in a thin layer or throughout.

The term covers a range of outcomes: titanium diffusion colours only a thin outer zone, while beryllium travels through an entire stone. Depth matters commercially, because shallow colour can be lost if a stone is repolished after damage, whereas deep diffusion is permanent. Diffused corundum sells for a small fraction of comparable heated-only material, and the treatment must be disclosed at every stage of sale.

Lead-glass composite

A lead-glass composite is heavily fractured, low-grade corundum whose cavities and fissures have been filled with high-lead-content glass, producing a manufactured material rather than a conventionally treated ruby.

Removing the glass would leave a fractured, near-opaque stone, which is why these are described as composites rather than treated rubies. Lemon juice, bathroom cleaners and a jeweller's pickle pot will etch the filling and leave visible white scars, and a repair torch can damage a stone in seconds. They sell for a small fraction of the price of heated ruby, and many jewellers decline to work on them.

Oiling

Oiling fills an emerald's surface-reaching fissures with a colourless oil of similar refractive index, so that the breaks reflect less light and become far less visible without any colour being added.

Almost every emerald on the market has been oiled, and laboratories grade the amount as none, minor, moderate or significant, which has a direct effect on price. Oil is not permanent: it can dry, seep out or be stripped by ultrasonic and steam cleaners, so an oiled emerald should be cleaned only in lukewarm soapy water. Re-oiling by a specialist is possible and is not regarded as damage.

See also: Cedarwood oil · Clarity enhancement · Jardin

→ The Emerald Buyer’s Handbook

Resin filling

Resin filling places a hardening polymer, rather than an oil, into a gemstone's fractures, giving a more durable result than oiling but one that is harder to reverse and may discolour.

Artificial resins such as Opticon hold better than oil under normal wear and do not seep out, but they are difficult to remove and some yellow with age. Parts of the trade discount resin-filled emeralds against oiled stones of the same appearance, so the substance used should be stated, not merely the degree of filling. Ultrasonic cleaning and torch heat still risk damaging the filler.

See also: Oiling · Fracture filling · Clarity enhancement

Simulant

A simulant imitates the look of a gemstone without sharing its chemical composition or crystal structure, as cubic zirconia does for diamond and green glass does for emerald.

The distinction from synthetic matters: a synthetic sapphire is corundum, whereas a simulant only looks like one. Simulants are usually straightforward to separate by refractive index, specific gravity, thermal or electrical conductivity, and often by eye once differences in lustre and dispersion are known. A simulant is worth what its own material is worth, which is generally a nominal amount.

Smoke treatment

Smoke treatment darkens an opal by enveloping it in smoke from burning organic matter, depositing a carbon layer that lowers apparent body tone and makes play-of-colour appear stronger.

Smoked opal, most often Ethiopian, is sometimes presented as black opal, but the darkening is a surface carbon layer rather than natural body tone, and a chip or repolish can reveal pale material beneath. Because Ethiopian opal is largely hydrophane, water can alter or partly lift the effect. A related method uses sugar and acid to deposit carbon; both must be disclosed and both reduce value sharply.

See also: Body tone · Opal · Treatment disclosure

Stabilisation

Stabilisation is the impregnation of a porous or fragile gem material with wax, resin or polymer so that it holds together well enough to be cut, set and worn.

Most commercial turquoise is stabilised, since untreated material of good hardness and colour is uncommon and soft turquoise absorbs oils and cosmetics until it darkens. Stabilisation genuinely improves wearability, so it is not in itself a mark of poor quality, though untreated natural turquoise commands a substantial premium. The same approach is applied to opal matrix, some coral and various soft ornamental stones.

See also: Dyeing · Turquoise · Treatment disclosure

Surface diffusion

Surface diffusion is a corundum heat treatment in which titanium or chromium is driven only a short distance into the stone, leaving a thin coloured skin that repolishing, chipping or wear can cut through.

Because the colour lives in a shallow skin, it concentrates along facet edges and shows as uneven patches when the stone is immersed in liquid. Any repolishing to remove a chip risks cutting through the coloured layer and exposing pale material beneath. Diffused blue sapphires and diffused star sapphires appear widely in low-cost jewellery and sit at the bottom of the corundum market.

Synthetic

A synthetic gemstone is grown in a laboratory but has essentially the same chemistry, crystal structure and optical properties as the natural stone it corresponds to, differing only in origin.

CIBJO retires the phrase laboratory-grown in favour of synthetic from September 2026, so terminology on reports and invoices will shift. Synthetic corundum, emerald, spinel and diamond are real forms of those minerals rather than imitations, and are grown by methods including flame fusion, flux, hydrothermal, HPHT and CVD. Identification rests on growth features and trace chemistry and sometimes needs a laboratory, so undisclosed synthetics remain a genuine risk.

See also: Simulant · Treatment disclosure

Treatment disclosure

Treatment disclosure is the seller's obligation to state in writing every process a gemstone has undergone beyond cutting and polishing, including treatments that are routine, industry-standard or difficult to detect.

CIBJO's rules and, in Australia, the consumer law prohibition on misleading conduct both make silence unacceptable; a treatment being common is not a reason to leave it out. The word natural describes origin, not condition, so a natural sapphire may still be heated. Ask for treatment status on the invoice or report before purchase; Ministry of Gems discloses treatment on every stone it sells.

→ Our treatment register

Unheated

Unheated describes a gemstone that has reached the market with its colour and clarity entirely natural, showing no laboratory evidence of thermal enhancement, a status confirmable only by gemmological report.

Laboratories rarely use the word unheated; the standard wording is no indications of heating, because absence of evidence is what testing can establish. Unheated stones with fine colour can trade at multiples of their heated equivalents, so the claim should never be accepted without a report from a recognised laboratory. Every stone at Ministry of Gems is graded in Sydney by an FGAA-qualified gemmologist before it is described.

→ Unheated sapphires in stock

→ Treatment register

Optical Phenomena & Appearance

Why a stone looks the way it does, and the words used to grade that look.

Adamantine lustre

Adamantine lustre is the hard, near-metallic surface brightness shown by diamond and by a small group of other gems with very high refractive index, generally those above about 1.9.

Diamond, at a refractive index of 2.42, sets the standard; zircon (roughly 1.93 to 1.99), demantoid garnet and sphene approach it and are usually called subadamantine. Corundum, at 1.76 to 1.77, is vitreous rather than adamantine, which is one quick way to distinguish a well-cut white sapphire from a diamond by eye. Lustre also depends on finish, so a worn or poorly polished diamond can look duller than its species allows.

See also: Lustre · Refractive index · Fire (dispersion)

Adularescence

Adularescence is the soft blue-white light that appears to float below the surface of moonstone, produced by the scattering of light from alternating microscopic layers of two intergrown feldspars.

The effect depends on those layers being thin enough to scatter shorter wavelengths, which is why the finest material shows a blue sheen over a near-colourless, transparent body rather than a white one. The cutter must dome the cabochon and orient it so the sheen sits centrally. Moonstone is only 6 to 6.5 on the Mohs scale and cleaves easily, so it suits pendants and earrings better than daily-wear rings.

See also: Chatoyancy · Play-of-colour · Lustre

Asterism

Asterism is the star effect seen in some cabochon-cut gems, where light reflecting from oriented needle-like inclusions forms a four- or six-rayed star that glides across the dome under a single light source.

In corundum the rays come from rutile silk aligned along three crystal directions at 120 degrees, so star sapphires and star rubies show six rays, while star garnet and black star diopside more often give four. The stone must be cut en cabochon and correctly oriented, or the star sits off-centre or fails to form. The same dense silk that creates the star also mutes colour and transparency.

See also: Chatoyancy · Colour zoning · Extinction

Birefringence

Birefringence is the difference between the highest and lowest refractive indices of a doubly refractive gem, a fixed figure that records how strongly the stone splits light into two rays.

Corundum is weakly birefringent at 0.008 to 0.010, while peridot reaches 0.035 to 0.038 and zircon 0.059, strong enough that a loupe shows the back facets apparently doubled. That doubling is a useful separation, because diamond, spinel and garnet are singly refractive and never show it. Birefringence is not a flaw and does not affect value, though a cutter may angle facets to keep visible doubling out of the face-up view.

See also: Refractive index · Pleochroism · Dichroism

Body tone

Body tone is the darkness or lightness of an opal's background, against which play-of-colour is seen, graded on the Australian N1 to N9 scale running from black through dark to light and white.

N1 to N4 is black opal, N5 to N6 dark or semi-black, and N7 to N9 light or white opal; 'crystal' describes transparency rather than tone and can occur across the range. Body tone is judged on the body of the stone rather than on the fire, and it is one of the principal drivers of opal price alongside brightness and pattern, because a darker background makes the same play-of-colour read more brightly. Lightning Ridge is the classic source of N1 to N4 material.

See also: Opal · Play-of-colour · Lightning Ridge

→ The Opal Buyer’s Handbook

Brilliance

Brilliance is the amount of white light a cut gem returns to the eye, produced by light entering the crown, reflecting from the pavilion facets and passing back out through the top.

It depends on refractive index, transparency and above all on cut, since the pavilion angles must hold light above the critical angle or it escapes through the back and leaves a window. Polish quality and an undamaged girdle matter as well. Because weight-retentive cutting trades angles for carats, two stones of identical species, size and colour can differ sharply in how much life they show.

See also: Windowing · Scintillation · Extinction

Chatoyancy

Chatoyancy, or the cat's-eye effect, is a single bright band of light that moves across a cabochon, caused by reflection from parallel fibrous inclusions or hollow tubes lying beneath the surface.

The trade reserves the unqualified name cat's eye for chrysoberyl (Mohs 8.5), the hardest and most valued of the eyed gems; every other species must be named, as in cat's-eye tourmaline or cat's-eye quartz. Value rests on how sharp, straight and centred the eye is, and on how cleanly it opens and closes as the stone is rocked. A milky body colour is common and lowers price.

See also: Asterism · Adularescence · Lustre

Colour zoning

Colour zoning describes the uneven distribution of colour within a gem, appearing as bands, patches or angular sectors that follow the growth structure of the crystal the stone was cut from.

Blue sapphire commonly shows straight or hexagonal bands of blue and colourless material, and amethyst is often zoned towards the crystal point. The cutter's task is to orient the stone so the colour reads evenly face up, which usually costs weight. Zoning still visible face up reduces value. Growth structure is also diagnostic: curved colour banding indicates flame-fusion synthetic corundum, while natural stones show straight or angular zoning.

See also: Saturation · Hue · Windowing

Dichroism

Dichroism is pleochroism in its two-colour form, shown by uniaxial gems such as corundum, tourmaline and zircon, whose ordinary and extraordinary rays absorb light differently and so give two distinct body colours.

A ruby typically reads purplish red in one direction and orangey red in the other, and the cutter chooses which dominates face up. Gemmologists observe it with a dichroscope, a small tube that sets the two images side by side; the effect is strongest at right angles to the optic axis and disappears entirely when a stone is viewed straight down it. A stone showing none in any direction is singly refractive. Dichroism is an identification aid rather than a value factor.

See also: Pleochroism · Birefringence · Refractive index

Extinction

Extinction is the blacked-out or dead area seen in a faceted gem, caused by pavilion angles cut too steeply, by a dark body tone, or simply by the observer's own head shadow.

A degree of extinction is normal and even useful, since the contrast between bright and dark facets is what the eye reads as sparkle. It becomes a fault when large zones stay dark as the stone is tilted, a common problem in deep cuts and in dark garnet, spinel and sapphire. Judge it face up in indirect daylight at arm's length. Gemmologists also use the word for the darkening seen in a polariscope, an unrelated laboratory sense.

See also: Tone · Windowing · Brilliance

Fire (dispersion)

Fire, properly called dispersion, is the splitting of white light into flashes of spectral colour inside a gem, which happens because each wavelength is bent by a slightly different amount.

Dispersion is a fixed property of the species: diamond measures 0.044, demantoid garnet 0.057 and sphene 0.051, while corundum sits low at 0.018, so sapphire and ruby are valued for colour rather than fire. Cut, polish and cleanliness decide how much of that potential is actually seen. In opal the trade also uses the word fire loosely for play-of-colour, which is diffraction and a different phenomenon entirely.

See also: Brilliance · Scintillation · Play-of-colour

Fluorescence

Fluorescence is visible light emitted by a gem while it is being exposed to ultraviolet or other higher-energy radiation, and it stops the instant the source is removed.

Chromium makes many rubies and red spinels glow red under long-wave ultraviolet and even in sunlight, which lifts their apparent colour in daylight, whereas iron-rich Thai rubies stay inert. Chalky blue-green fluorescence under short-wave ultraviolet can point to heat treatment in sapphire, so laboratories check it routinely. In diamond, blue fluorescence occurs in roughly a quarter to a third of stones and is a description rather than a defect, although very strong grades occasionally look hazy.

See also: Colour zoning · Hue · Saturation

Hue

Hue is the basic spectral position of a gem's colour, the attribute that makes it red, blue or green, and it is normally written with the secondary hue first, as in violetish blue.

Trade preference settles on particular hues within each species: pure to slightly violetish blue in sapphire, red to slightly purplish red in ruby, and pinkish orange in padparadscha, which is a colour category of sapphire rather than a separate species. Greenish modifiers usually reduce price in both sapphire and ruby. Hue must be judged face up in indirect daylight, because the light source changes what the eye reads.

See also: Saturation · Tone · Pleochroism

Lustre

Lustre is the appearance of light reflected from a gem's surface, described by terms such as vitreous, adamantine, resinous, waxy, silky or pearly rather than by any numerical scale.

It is governed chiefly by refractive index and by how well the surface takes and keeps a polish, which in turn depends on hardness. Corundum and beryl are vitreous, or glass-like, diamond is adamantine and zircon nearly so, while pearl has its own nacreous lustre and, at 2.5 to 4.5 on the Mohs scale, dulls easily. A faceted stone that has lost its polish can be repolished at the cost of a little weight; a pearl cannot, because its nacre is thin and wears away permanently.

Play-of-colour

Play-of-colour is the shifting display of spectral flashes seen in precious opal, produced when light diffracts through a regular three-dimensional array of microscopic silica spheres inside the stone.

It is the only reason precious opal carries value; common opal has the same chemistry but no ordered sphere structure and therefore no play. Brightness, the range of colours present and the pattern all drive price, with red over a dark body tone the most sought. Assembled doublets and triplets show play-of-colour yet are worth far less than solid opal, and can separate if soaked.

See also: Tone · Fire (dispersion) · Lustre

→ The Opal Buyer’s Handbook

Pleochroism

Pleochroism is the property by which a doubly refractive gem shows different body colours along different crystal directions, appearing as two colours in uniaxial species and three in biaxial ones.

Tanzanite is the textbook case, showing blue, violet and a brownish or burgundy direction within the same piece of rough, so orientation decides both the face-up colour and how much weight the cutter must give away. Iolite and andalusite behave similarly. Singly refractive gems such as diamond, spinel and garnet show no pleochroism at all, and a dichroscope uses this difference to separate species in seconds.

See also: Dichroism · Birefringence · Hue

Refractive index

Refractive index measures how much a gem slows and bends light entering it from air, and the resulting figure is one of the most reliable identifying properties available to a gemmologist.

A refractometer reads it in seconds: corundum sits at 1.762 to 1.770, spinel at about 1.717, beryl at 1.577 to 1.583 and quartz at 1.544 to 1.553. Standard refractometers stop near 1.81, so diamond and zircon read as over-the-limit and require other tests. A higher refractive index allows greater potential brilliance and lustre, though only a well-proportioned cut will realise it.

See also: Birefringence · Lustre · Brilliance

Saturation

Saturation measures the intensity or purity of a gem's hue, meaning how far the colour stands away from grey in the cool hues and away from brown in the warm ones.

Together with tone it is the strongest influence on price in coloured stones, and a vividly saturated sapphire will trade far above a greyish one of the same size and origin. Laboratories generally grade it on a six-step scale running from greyish or brownish through to vivid. Saturation should be assessed in indirect daylight, since a spotlit display case exaggerates it and makes almost any stone look richer than it is.

Scintillation

Scintillation is the pattern of sparkle a faceted gem gives off as the stone, the light source or the observer moves, created by facets flashing alternately bright and dark.

It is distinct from brilliance, which is the steady return of white light, and from fire, which is spectral. Facet size, number, symmetry and polish govern it: many small facets produce fine, busy sparkle, while fewer large ones give broad flashes. Scintillation is best judged by rocking the stone in indirect daylight rather than under the spotlights of a display counter, which flatter almost any gem.

See also: Brilliance · Fire (dispersion) · Extinction

Tone

Tone is the lightness or darkness of a gem's colour, graded from 0 for colourless to 10 for black, with medium to medium-dark readings generally the most sought in coloured stones.

Too light and a stone looks washed out; too dark and it closes into extinction, which is why very dark blue sapphire and some garnets are difficult to sell despite strong saturation. Tone interacts with cut, since a deep pavilion darkens colour and a shallow one lightens it. Opal uses a separate body tone scale running N1 for black to N9 for white, and the two should not be confused.

See also: Saturation · Extinction · Hue

Windowing

Windowing occurs when the pavilion of a faceted gem is cut too shallow, so light passes straight out through the back instead of reflecting, leaving a see-through area in the centre of the stone.

Hold the gem over printed text; if the type can be read through the middle, it has a window. Windows arise where rough is cut for maximum weight retention, and they cost both brilliance and apparent depth of colour, so the stone looks paler and larger than its carat weight suggests. Recutting can close a window, but it always removes weight, which is why the fault is best judged before purchase rather than corrected afterwards.

See also: Brilliance · Extinction · Tone

Cut & Stone Anatomy

The parts of a cut stone and the shapes cutters work in.

Brilliant cut

The brilliant cut arranges triangular and kite-shaped facets radiating from the centre to return the maximum light through the table; a round brilliant has 57 facets, or 58 when a culet facet is polished on.

These proportions were worked out for diamond, where the aim is maximum white light return and fire. Applied to coloured stones the outcome varies: the style flatters lighter material such as pale sapphire or aquamarine, but on darkly saturated rough it can push a gem towards extinction. Facet counts are often quoted in selling copy, yet angles and symmetry are what determine whether a brilliant actually performs.

See also: Step cut · Mixed cut · Facet

→ The Engagement Ring Handbook

Cabochon

A cabochon is a gemstone finished with a smooth domed top and a flat or gently curved base instead of facets, the standard form for opal, star sapphire and moonstone.

Cabochon is not a lesser treatment. It is the only sensible cut for opal, and it is required for optical phenomena, since the star of a star sapphire or the eye of a cat's-eye chrysoberyl only forms across a curved dome. Dome height matters: too flat and a star sits weak and diffuse, too high and colour darkens. Heavily included material is also cut this way, so judge the individual stone rather than the style.

See also: Asterism · Chatoyancy · Body tone

Calibrated stone

A calibrated stone is cut to a standard commercial millimetre size, such as a 7 x 5 mm oval or a 2 mm round, so that it fits ready-made mountings without alteration.

Calibration exists because commercial settings are manufactured to fixed dimensions, so a calibrated gem drops into thousands of mounts. The cost is that the cutter works to a millimetre target rather than to the best proportions the rough allows, which can produce a shallow, windowed stone. Free-size and precision-cut gems often perform better optically but need a setting built around them and are harder to replace like for like.

Crown

The crown of a faceted gemstone is the upper section between the table and the girdle, where returning light is broken into the flashes seen as brilliance and fire.

Crown height and angle govern how much light is dispersed before it leaves the stone. A shallow crown gives a flat, lifeless face; an unusually tall one adds weight above the girdle, sits high in a mount and catches on clothing. In coloured gems a well-proportioned crown also lifts colour, because light passes twice through the body of the stone before reaching the eye.

Culet

The culet is the point or small facet at the very bottom of a faceted gemstone, where the pavilion facets converge.

Most modern stones close to a fine point, and a bare point chips easily, so a well-made setting shields it. Older European and Ceylon cuts frequently show a large open culet, visible through the table as a small circle that buyers sometimes mistake for a chip or an inclusion. It is neither, although a very large culet does weaken light return through the centre of the stone.

See also: Pavilion · Cushion cut · Facet

Cushion cut

The cushion cut is a square or rectangular outline with rounded corners and gently curved sides, resembling a pillow, and it may be faceted in brilliant, step or mixed style.

The cushion is the traditional outline for Ceylon sapphire, partly because it follows the shape of much rough and preserves weight. Antique cushions, cut before electric light, tend to have small tables, high crowns and large culets, and they glow rather than sparkle; modern versions are brighter and more uniform. Length-to-width varies widely under the same name, so look at the actual outline rather than the label.

See also: Mixed cut · Culet · Native cut

→ Design your ring

Emerald cut

The emerald cut is a rectangular step cut with bevelled corners, a shape developed to protect brittle emerald from chipping at the corners during setting and wear.

The cropped corners are functional rather than decorative: emerald is brittle because of its inclusions despite a hardness of 7.5 to 8, and a sharp corner is the first thing to break. The style is now used across every species. Because a step cut is so open, a shallow emerald cut readily shows a window through the table, so view the stone face-up in indirect daylight before committing.

See also: Step cut · Windowing · Oiling

A facet is one of the flat, polished planes cut onto the surface of a gemstone, angled and arranged so that light entering the stone is reflected back to the eye.

Facets work as internal mirrors: light enters through the crown, strikes the pavilion facets and returns to the viewer. Their angles matter far more than their number, and a high facet count is not a measure of quality, since a well-angled 57-facet stone will outperform a poorly proportioned one with a hundred. Facet edges soften with decades of wear and can be repolished by a cutter.

See also: Table · Pavilion · Brilliant cut

Girdle

Encircling the widest part of a faceted gemstone, the girdle is the narrow band that separates crown from pavilion and provides the edge a setting grips.

The girdle is the stone's most exposed edge and the point where prongs and bezels take hold. A knife-thin girdle chips readily, especially on pointed outlines and on species softer than corundum, while an over-thick one adds weight without adding face-up size. Girdles may be left frosted, polished or faceted, and none of those finishes affects durability. On diamonds it is also where laboratory inscriptions are placed.

Marquise cut

The marquise cut, also called the navette, is an elongated boat-shaped stone pointed at both ends, a design that maximises face-up size for its weight.

Two points mean two places to chip, so V-tips or a bezel are sensible protection. Marquises use elongated rough efficiently and read as long and slender on the hand, but weaker examples show a pronounced bow-tie and can look thin and glassy towards the tips. Length-to-width around 1.75 to 2.25 to one is the usual range; much longer and the ends become fragile.

See also: Pear cut · Oval cut · Girdle

Melee

Melee is the trade term for small faceted gemstones, conventionally those under about 0.18 carat, used in quantity as accent stones in pavé, halo and channel settings.

Melee is bought in parcels and matched for colour, size and cut, and it does much of the visual work in halos and eternity bands. It is also where cost is quietly cut, because buyers seldom examine stones this small: uneven sizing, poor polish and undisclosed synthetic or treated material have all appeared in melee parcels. Ask what the accent stones are and on what basis they were bought.

See also: Calibrated stone · Facet · Synthetic

→ Design your ring

Mixed cut

A mixed cut combines two faceting styles on one stone, typically a brilliant-faceted crown above a step-cut pavilion, balancing light return at the top with colour and weight retention below.

Most coloured stones on the market are mixed cuts, and it is a considered choice rather than a compromise. The brilliant crown supplies sparkle while the step-cut pavilion holds depth and colour and keeps more of the original rough. Sri Lankan cutters have worked this way for generations. Judge the result rather than the label, since a mixed cut can still run too deep, and excess depth is weight you cannot see face-up.

See also: Brilliant cut · Step cut · Native cut

Native cut

Native cut describes a gemstone faceted close to its source, in Sri Lanka, Thailand or East Africa, where the cutter has favoured retaining weight from the rough over ideal proportions.

The incentive is structural rather than careless, since rough is bought and sold by weight. Results are uneven: deep pavilions, off-centre culets, lopsided outlines and windows are all common, though a deep native-cut Ceylon sapphire can also hold colour very well. Ministry of Gems recuts stones in its own workshop where the improvement justifies the loss. Assess each stone face-up rather than by the reputation of the term.

See also: Recut · Pavilion · Mixed cut

Oval cut

An oval cut is an elongated elliptical outline, most often faceted as a brilliant, that presents a larger face-up area than a round stone of the same carat weight.

Ovals show more surface for their weight, which is why a 2 carat oval usually looks bigger than a 2 carat round. The trade-off is the bow-tie, a dark band across the centre caused by pavilion angles and by the viewer's own shadow. Nearly every oval has one to some degree; the question is whether it is faint or dominant. Tilt the stone in indirect daylight to judge it honestly.

See also: Brilliant cut · Pear cut · Pavilion

Pavilion

The pavilion is the lower portion of a faceted gemstone, below the girdle, whose angled facets reflect light back up through the crown instead of letting it escape through the base.

Pavilion depth decides light return. Too shallow and light passes straight through, leaving a pale window in the centre; too deep and the stone darkens under the table while carrying weight you pay for but cannot see face-up. Cutters of coloured stones deliberately leave pavilions deeper than diamond proportions because extra depth saturates colour, which is a legitimate choice that becomes a fault only when taken too far.

Pear cut

Combining a rounded end with a single point, the pear cut, also called the pendeloque or teardrop, is an elongated brilliant shape used widely for pendants and drop earrings.

The point is the weak spot and belongs in a V-shaped prong or a protective bezel. Check symmetry: the two shoulders should mirror each other and the point should line up with the centre of the rounded end, because a crooked pear becomes obvious once set. Pears share the oval's tendency to show a bow-tie, and they are a common way to keep weight from elongated rough.

See also: Oval cut · Marquise cut · Girdle

Recut

A recut is the refaceting of an already finished gemstone to correct its proportions, remove damage or change its outline, and it always costs some of the original weight.

Recutting is worth considering when a stone has a large window, a chipped girdle or a badly off-centre culet, and the improvement in light return can be substantial. It is also irreversible, and a loss of ten to thirty per cent of the weight is normal, which matters if it takes the gem below a round figure such as 1.00 carat where pricing steps. Any existing laboratory report no longer describes the stone and must be reissued.

Step cut

A step cut is faceted in rows of long, parallel facets running around the stone like descending steps, producing broad flashes of colour rather than the fine sparkle of a brilliant.

Step cuts hide nothing. With few facets and long open planes they reveal colour zoning, inclusions and weak polish that a brilliant would break up, which is why cutters reserve them for clean, evenly coloured material. In return they give a calm, architectural look and long flashes across the table. The emerald and baguette cuts are the familiar examples, and both suit fine saturated colour.

See also: Emerald cut · Brilliant cut · Colour zoning

Table

The table is the large flat facet on the top of a faceted gemstone, the main window through which light enters and returns to the eye.

Table size is quoted as a percentage of the stone's width and sets the balance between the broad flash off the table and the smaller sparkle from the crown facets. An oversized table can leave a gem looking flat or glassy, while a very small one darkens the face-up view. The table is also where a window or extinction shows most plainly, so assess any coloured stone table-up in indirect daylight.

Trilliant

A trilliant is a triangular cut, with straight or gently convex sides and brilliant-style faceting, used both as a centre stone and in matched pairs flanking a larger gem.

The names trilliant, trillion and trillian are used interchangeably in the trade. Trilliants are usually cut shallow, so they look generous for their weight but are prone to windowing, and the three corners chip if left unprotected. Because outlines range from straight-sided to strongly curved, a replacement trilliant rarely matches an existing one without recutting, which is worth knowing before a repair.

See also: Brilliant cut · Calibrated stone · Recut

Clarity & Inclusions

What is inside a stone, what it means, and when it matters.

Cleavage

Cleavage is a mineral's tendency to split along flat planes of weak atomic bonding; diamond and topaz have it, beryl only weakly, and corundum has none at all.

Cleavage is a durability property rather than an appearance one, and it is set by the mineral species, not by the individual stone. Topaz has one perfect basal cleavage, which is why a knock in the wrong direction can take a clean slice off a topaz that has worn well for years. Cutters orient facets away from cleavage planes, and settings for cleavable stones should protect the girdle and corners.

See also: Parting · Fracture · Toughness

Eye-clean

Eye-clean describes a gemstone in which no inclusion is visible to normal unaided vision, viewed face-up at roughly 15 to 30 centimetres, even though magnification will still reveal internal features.

Eye-clean is a trade description, not a graded standard, and sellers apply it at different distances and under different lighting, so it is fair to ask what viewing distance was used. It is a reasonable expectation in aquamarine or tanzanite, a fair one in sapphire, and an unusual one in emerald. Assess stones in indirect daylight rather than under a bright spotlight, which can hide inclusions that reappear in ordinary room light.

See also: Loupe-clean · Type II gemstone · Inclusion

Feather

Feather is the trade name for a fracture or partly healed break inside a gemstone that catches light as a whitish, wing-like plane when the stone is tilted.

The word is descriptive rather than reassuring: a feather is a break, and its significance depends on size, orientation and whether it reaches the surface. Internal feathers well away from the girdle are usually harmless, but one that breaks the surface can catch during setting, extend under a jeweller's tools, or admit cleaning fluids. Anyone considering a stone with a large feather should ask for it to be pointed out under magnification.

See also: Fracture · Cleavage · Inclusion

Fingerprint inclusion

A fingerprint inclusion is a partly healed fracture in which trapped fluid has gathered into curved rows of tiny droplets, producing a rippled pattern that resembles a human fingerprint.

Fingerprints are strong evidence of natural growth in corundum and spinel, and gemmologists read their texture closely, since heating alters them and a rounded, beaded or broken-up fingerprint can indicate a stone has been through the furnace. They usually sit wholly within the gem rather than breaching the surface, so they seldom raise a durability concern. Flux-grown synthetics can imitate them, which is why a laboratory weighs the whole inclusion scene rather than one feature.

Fracture

A fracture is a break through a gemstone that ignores the crystal's internal planes of weakness, leaving an irregular or shell-like conchoidal surface rather than a flat one.

Fractures matter more for wear than for appearance: a surface-reaching fracture is where a stone can chip, and it is also the route by which oils, resins and lead glass enter treated material. Quartz, beryl and corundum all fracture conchoidally. A jeweller should be told about any surface-reaching fracture before setting or resizing, since heat and ultrasonic cleaning can extend it.

See also: Cleavage · Feather · Fracture filling

Halo inclusion

A halo inclusion is a disc-shaped stress fracture encircling a small crystal inside a gemstone, formed where the enclosed crystal expanded against its host and cracked the surrounding material.

Peridot's version, a brownish chromite flake ringed by a flat disc, is known in the trade as a lily pad. In corundum, zircon crystals commonly sit inside halos formed naturally, as radioactive decay expands the zircon over geological time, so a halo alone is not evidence of heating; laboratories judge treatment by whether the trapped crystal itself looks altered or whitened. Halos are usually internal and harmless, though a large one beneath the table scatters light and dulls brightness.

See also: Inclusion · Fracture · Heat treatment

Inclusion

An inclusion is any foreign crystal, cavity, fracture or growth feature enclosed within a gemstone, and it serves gemmologists as evidence of the stone's identity, natural or synthetic origin, and treatment history.

Inclusions are the ordinary condition of natural gemstones rather than a fault, and their number, size, position and visibility drive a large part of price. What matters commercially is whether they are visible face-up and whether they threaten durability: a fine needle near the girdle is a different proposition from a fracture that reaches the surface. Under magnification they also act as a fingerprint of origin, which is why laboratories photograph them.

See also: Jardin · Feather · Eye-clean

Jardin

Jardin, French for garden, is the trade term for the mossy network of inclusions and healed fissures found in almost every emerald, so named because it resembles foliage under magnification.

Emerald is judged by different clarity expectations from other gems: a jardin is normal, and a wholly clean emerald is rare enough to invite questions about whether it is synthetic. Buyers should consider how visible the jardin is face-up, how much it dulls transparency, and whether any fissure reaches the surface, since surface-reaching breaks carry both filling and durability implications.

See also: Type III gemstone · Feather · Inclusion

→ The Emerald Buyer’s Handbook

Loupe-clean

Loupe-clean means no inclusions can be seen in a gemstone under a 10x loupe by a trained observer, a considerably stricter standard than eye-clean and uncommon in ruby, sapphire and emerald.

In coloured stones, loupe-clean material is scarce enough to carry a premium and, in emerald or ruby, scarce enough to warrant confirming the stone is not synthetic. The term is borrowed from diamond grading, where the closest equivalents are the flawless and internally flawless grades assessed at the same magnification. Because 10x is the agreed standard, a claim made with a 20x or 30x scope is not the same statement.

See also: Eye-clean · Inclusion · Type I gemstone

Needle inclusion

Needle inclusions are long, slender crystals or hollow tubes trapped inside a gemstone, usually visible only under magnification and commonly aligned with the directions of the host crystal.

Needles are among the most common inclusions in corundum, beryl, spinel and quartz, and on their own they rarely affect durability. Their effect on value depends on density and position: scattered needles in the pavilion may go unnoticed, while a dense field reads as haze and dulls the stone face-up. Identifying the needle mineral, such as rutile, apatite or actinolite, often helps a gemmologist narrow origin.

See also: Rutile silk · Inclusion · Eye-clean

Negative crystal

A negative crystal is a hollow cavity inside a gemstone whose walls follow the host mineral's own crystal faces, typically filled with gas, liquid, or both.

Because the void copies the geometry of the host, a negative crystal is a reliable natural growth feature and is common in sapphire, spinel and quartz. It has no effect on durability unless it is large or sits close to the surface, where cutting or repolishing could open it. Under the loupe it can be mistaken for a gas bubble, so its angular outline is the distinguishing detail.

Parting

Parting is a break along planes of structural weakness created by twinning or by layers of included material, and unlike cleavage it occurs only in specimens that carry those features.

Corundum has no cleavage but frequently shows parting along twin planes, which is why ruby and sapphire wear extremely well yet can still split under a sharp blow at the wrong angle. Heavily twinned material, including some Ceylon and basaltic Australian sapphire, is the usual context. For a wearer the practical consequence is small, but a cutter or setter working a strongly parted stone will take extra care with heat and pressure.

See also: Cleavage · Fracture · Twinning

Rutile silk

Rutile silk is the fine, intersecting network of titanium dioxide needles inside corundum that scatters light as a soft whitish sheen and, when dense and correctly oriented, produces a star.

Silk is one of the clearest indications that a sapphire or ruby is natural and unheated, because heat treatment dissolves fine rutile into the host. In small quantities it can soften and spread colour across the stone; in excess it clouds the gem and reduces brightness. Star sapphires depend on silk entirely, so a cabochon is cut to place it correctly rather than to remove it.

See also: Needle inclusion · Asterism · Heat treatment

Toughness

Toughness is a gemstone's resistance to chipping, cracking and breaking under impact, a property separate from hardness and often the better guide to whether a stone will survive daily wear.

Jade is the classic example: softer than quartz on the Mohs scale, yet its interlocking structure makes it exceptionally difficult to break. Emerald sits at the opposite extreme, hard at 7.5 to 8 but internally fractured and brittle. Cleavage planes, existing fractures and large included crystals all reduce toughness, which is why a setting that protects the girdle can matter more than the hardness figure alone.

See also: Mohs scale · Cleavage · Jardin

Twinning

Twinning is the symmetrical intergrowth of two or more segments of the same crystal, common in corundum, where repeated twin planes create internal directions of weakness within an otherwise very hard stone.

Repeated, or polysynthetic, twinning in corundum produces flat lamellae running through the crystal, and parting frequently develops along them — a plane that behaves like cleavage even though corundum has none. Cutters and setters orient work away from these planes, because a knock aligned with one can split a stone that would otherwise resist almost anything. Under magnification twinning shows as a fine grid of intersecting straight lines, which a gemmologist reads as growth structure rather than damage.

Two-phase inclusion

A two-phase inclusion is a sealed cavity within a gemstone holding two states of matter, most often a liquid with a gas bubble; add a solid crystal and it becomes three-phase.

These are growth features and good evidence of natural origin, since the fluid was trapped as the crystal formed. Three-phase inclusions of liquid, gas bubble and salt crystal are strongly associated with Colombian emerald and are one of the features a laboratory weighs when forming an origin opinion. They are stable in ordinary wear, but a two-phase inclusion lying close to the surface can be damaged by steam cleaning or by heat during repairs.

Type I gemstone

Type I gemstones are varieties that typically grow free of inclusions visible to the unaided eye, among them aquamarine, morganite, blue topaz, tanzanite and blue zircon, so eye-clean is the normal expectation.

The three clarity types are a grading framework rather than a quality ranking: they set what a buyer should reasonably expect of that material before judging an individual stone. Because Type I gems are usually clean, a visible inclusion counts against one far more heavily than the same inclusion would in a Type II or Type III gem. Pricing follows the same logic, so eye-visible flaws in aquamarine or tanzanite are discounted sharply.

Type II gemstone

Type II gemstones usually contain some inclusions visible to the unaided eye, a group that includes sapphire, ruby, spinel, garnet, peridot, iolite, amethyst and most tourmaline.

Most of the coloured-stone market sits in Type II, where inclusions are the norm rather than a defect: a fine sapphire is judged first on colour, then on how much its inclusions interfere face-up. Eye-clean Type II material exists, but the premium it carries rises steeply with size. Position matters as much as quantity, since an inclusion beneath the table is more disruptive than the same feature near the girdle.

See also: Type I gemstone · Type III gemstone · Inclusion

Type III gemstone

Type III gemstones almost always contain inclusions visible to the unaided eye, chiefly emerald, red beryl and pink to red tourmaline, so a clean example is the exception rather than the standard.

With Type III material the trade relaxes its clarity expectations and asks instead whether the inclusions have cost the stone its transparency and colour. Emerald is the clearest case: nearly all are oiled or resin-filled to reduce the visibility of surface-reaching fissures, and that treatment should be disclosed with its grade of none, minor, moderate or significant. A visibly included emerald with fine colour commonly trades above a cleaner but paler one.

See also: Jardin · Type II gemstone · Fracture filling

Origin, Geology & Fields

Where gems form, and the named fields the trade trades on.

Alluvial deposit

An alluvial deposit is a secondary gem occurrence in which crystals weathered out of their host rock have been carried and concentrated by running water into river gravels.

Water sorting does the first grading for the miner: fragile or heavily fractured material tends to break up in transport, so alluvial gravels are often enriched in tougher, cleaner stones. Sri Lanka's illam gravels around Ratnapura are the classic example and still supply much of the world's fine sapphire. The trade-off is that an alluvial stone has left its host rock, which is one reason the precise geological source of a parcel can be difficult to establish.

See also: Placer deposit · Primary deposit · Ratnapura

Anakie

Anakie is the historic gem centre of the Central Queensland sapphire fields, a basalt-sourced district that has produced golden yellow, green, parti-coloured and black star sapphire since the late nineteenth century.

The Queensland fields, taking in Anakie, Sapphire, Rubyvale and the Willows, are best known for warm honey and golden yellows and for strongly parti-coloured crystals, alongside the dark blues typical of basaltic sources. Queensland black star sapphire, a six-rayed star over a near-opaque body, is a signature of the district. Much Queensland rough was historically exported for cutting and heat treatment, so older stones sold without paperwork are frequently Australian in fact if not in name.

See also: Rubyvale · Basalt field · Glen Innes

Basalt field

Basalt fields are sapphire-producing districts where alkali basalt magma carried already-formed corundum crystals to the surface, leaving them to weather out into the surrounding soils and creek gravels.

Eastern Australia, Thailand, Cambodia, Nigeria and parts of Madagascar all sit on basaltic sapphire fields. Their stones tend to be iron-rich, which pushes colour towards deep blue, teal, green and golden yellow, and can make larger stones read as dark or inky in weak light. Strong colour zoning and parti-coloured crystals are common, and basaltic rough has long been heat-treated to lift the blues, so heating should be assumed unless a laboratory says otherwise.

See also: Inverell · Anakie · Placer deposit

Chivor

Chivor is one of Colombia's two classic emerald districts, lying in the eastern belt of Boyacá, and its stones tend towards a slightly bluish green, cooler and often lighter than classic Muzo material.

That colour difference is a tendency rather than a rule, and the colour ranges of the two belts overlap so far that no laboratory can reliably separate Chivor from Muzo; a report states Colombia rather than naming a mine. Chivor emeralds are often described as cleaner on average, though inclusions are expected in all emerald and nearly every stone has been oiled or resin-filled to some degree. Ask for a stated clarity-enhancement grade of none, minor, moderate or significant.

See also: Muzo · Origin determination · Jardin

→ Emeralds in stock

Coober Pedy

Coober Pedy is a South Australian opal field known for light and white opal and for transparent crystal opal, and for a township where much of the population lives in underground dugouts.

Opal from the field usually falls in the N7 to N9 body-tone range, where play-of-colour sits against a pale background and reads more softly than the same colour would on black. Crystal opal, transparent to semi-transparent with colour running through the body rather than across a surface, is the district's most valuable material. Light opal is an accessible entry to solid Australian opal, but at 5.5 to 6.5 on the Mohs scale all opal needs protective settings and no ultrasonic cleaning.

See also: Lightning Ridge · Body tone · Opal

→ Opals in stock

Elahera

Elahera is a gem field in central Sri Lanka, in the Matale District north-east of Kandy, worked commercially from the 1970s for blue and yellow sapphire, chrysoberyl, alexandrite and spinel.

The field lies within the same Highland Complex metamorphic belt as Ratnapura but produces a somewhat different mix, with a reputation for strong blue sapphire and for fine cat's eye chrysoberyl. Gem-bearing ground here is often deeper than in the south-west, so pits are larger and more heavily timbered. Note that laboratories issue country-level origin opinions for Sri Lanka; no report can confirm Elahera specifically as against any other Sri Lankan field.

Glen Innes

Glen Innes, in the New England highlands of New South Wales, is the second focus of Australia's eastern sapphire field, producing blue, green, yellow and parti-coloured stones from wash derived from weathered basalt.

Glen Innes and Inverell work the same broad geology less than a hundred kilometres apart, and rough from the two towns is generally traded together as New England sapphire. The district also yields zircon and, occasionally, black star sapphire. Public fossicking areas keep the field open to visitors, which is unusual among the world's gem sources. The practical consequence of small-scale, intermittent supply is that matched suites of Australian sapphire are hard to assemble.

See also: Inverell · Basalt field · Placer deposit

Inverell

Inverell is a town in the New England district of northern New South Wales, at the centre of a basalt-derived sapphire field that supplied a large share of world blue sapphire rough during the 1970s and 1980s.

Most Inverell rough left Australia unpolished and was cut and sold through Thailand, so a good deal of unattributed blue sapphire in older jewellery is in fact Australian. The stones run deep blue to blue-green, parti-coloured blue and yellow crystals are common, and the iron content gives a directness of colour that some buyers prefer to lighter Ceylon blues. Commercial output is far below its peak, and much of the field is now worked by small operators.

See also: Glen Innes · Basalt field · Anakie

Kashmir sapphire

Kashmir sapphire denotes blue sapphire from the Padar area of the Zanskar Range in the Indian Himalaya, mined intensively during the 1880s and prized for a velvety, slightly sleepy blue.

That texture comes from clouds of microscopic inclusions which scatter light and soften the colour, so the stone holds its blue across lighting conditions instead of swinging between bright and dark. Meaningful production ended within a few years of discovery, and effectively all Kashmir sapphire on the market is recirculated old stock. The name commands extreme premiums, so it should never be accepted without an origin opinion from a leading laboratory, and even then laboratories can disagree.

See also: Origin determination · Pegmatite · Mogok

→ Sapphire origin guide

Lightning Ridge

Lightning Ridge, in north-western New South Wales, is the world's principal source of black opal, meaning precious opal of dark body tone N1 to N4 against which play-of-colour reads at its strongest.

Opal here formed in Cretaceous claystone and sandstone of the Great Artesian Basin, and the district is the only significant producer of nobbies, rounded nodules of solid opal. Dark body tone carries the price: an N1 stone with strong red play-of-colour sits at the top of the opal market, well above equivalent light opal. Confirm that a stone is solid rather than a doublet or triplet, since assembled stones are worth far less and can separate if soaked.

See also: Body tone · Coober Pedy · Doublet

→ Opals in stock

Metamorphic deposit

Metamorphic deposits form where heat and pressure recrystallise existing rock, yielding gems such as marble-hosted ruby and spinel, and the sapphire and chrysoberyl of Sri Lanka's high-grade gneisses.

The geology has direct consequences for appearance. Marble is poor in iron, so rubies grown in it at Mogok, Luc Yen and Jegdalek fluoresce strongly in daylight and glow rather than darken, while iron-bearing amphibolite and basalt sources give quieter reds. Sri Lanka's Highland Complex, metamorphosed at granulite grade, is unusual in producing more than thirty gem species from one terrain, padparadscha and fine cat's eye chrysoberyl among them.

See also: Mogok · Primary deposit · Basalt field

Mogok

Mogok, the valley in northern Myanmar known in the trade as the Valley of Rubies, is the classic source of marble-hosted ruby and also yields fine spinel, peridot and sapphire.

Marble is very low in iron, so Mogok rubies fluoresce red in daylight and appear to glow rather than darken, which is the look behind the term pigeon's blood, itself a laboratory colour opinion rather than an origin or a guarantee. Burmese origin attracts a substantial premium, so a report from a reputable laboratory is essential. Sanctions and import restrictions on Burmese stones differ by country and change over time.

→ Rubies in stock

Montepuez

Montepuez in northern Mozambique is an amphibolite-hosted ruby field discovered in 2009 and developed into large-scale production, now supplying a substantial share of the world's fine ruby.

Its arrival reshaped the ruby market, ending decades in which Burmese material dominated supply of large, comparatively clean stones. Montepuez rubies carry more iron than marble-hosted Burmese stones, so they fluoresce less and can appear deeper or more crimson, while the finest reach colour that laboratories will describe as pigeon's blood. A meaningful proportion is sold unheated with laboratory confirmation, and traceability from mine to market is better documented than for most ruby sources.

See also: Mogok · Origin determination · Primary deposit

→ Rubies in stock

Muzo

Muzo is the historic emerald district of Colombia's Boyacá department, worked since pre-Columbian times and associated with a warm, slightly yellowish green widely treated as the benchmark for fine emerald colour.

Colombian emerald is geologically unusual, having grown in calcite and pyrite veins within black shale rather than alongside the granite or pegmatite that hosts emerald almost everywhere else, and the stones are low in iron with a saturated green. Three-phase inclusions, a cavity holding liquid, a gas bubble and a salt crystal, point towards Colombia, though similar inclusions occur in Afghan and Chinese material, so laboratories weigh them alongside trace-element chemistry. Muzo green is often used loosely as a colour description rather than a claim of mine origin.

See also: Chivor · Origin determination · Jardin

→ Emeralds in stock

Origin determination

Origin determination is the laboratory process of forming an opinion on a gem's geographic source by comparing its inclusions, trace-element chemistry and spectra against reference stones of known provenance.

It produces an opinion, not a measurement: confidence depends on how distinctive a deposit is and how well it is represented in a laboratory's reference collection, so two reputable laboratories can reach different conclusions on the same stone. Because names such as Kashmir, Burma and Colombia carry very large premiums, an origin claim unsupported by a report from a laboratory with a strong record in origin work should be read as a seller's view. Heating does not change origin, but it can obscure the inclusion evidence the work relies on.

See also: Kashmir sapphire · Muzo · Primary deposit

→ Sapphire origin guide

Pegmatite

Pegmatite is an exceptionally coarse-grained igneous rock, crystallised from the last water-rich fraction of a cooling magma, and it is the classic host for tourmaline, aquamarine, morganite and topaz.

Because that residual fluid is rich in water and in rare elements such as beryllium, boron and lithium, pegmatites grow large, well-formed crystals, which is why clean aquamarine and tourmaline are available in sizes that would be extraordinary in ruby or emerald. Durability varies within the group: tourmaline sits at 7 to 7.5 on the Mohs scale, while topaz is 8 but carries a perfect basal cleavage that demands care in setting and wear.

See also: Primary deposit · Kashmir sapphire

Placer deposit

Placer deposit is the general geological term for any accumulation of durable, heavy minerals, gems among them, concentrated by gravity and moving water or wind after erosion from their original rock.

Alluvial river placers are the best-known kind, but the family also takes in eluvial deposits sitting just downslope of the source, colluvial slope wash, and marine or beach placers. Gems survive that journey because they are hard and chemically stable, which is why placer mining historically found the toughest species first. Most Sri Lankan production, and nearly all Australian sapphire reaching the market, comes from placer ground rather than hard rock.

Primary deposit

A primary deposit is one where gem crystals still sit in the rock in which they crystallised, so they are mined in situ rather than recovered from gravels.

Marble-hosted ruby at Mogok, emerald in the calcite veins of Colombia's black shales and aquamarine still in its pegmatite pocket are all primary occurrences. Sapphire in basalt is not: the magma only carried crystals that had already grown at depth. Working primary ground means moving rock rather than washing gravel, which is slower and more capital-intensive, and crystals often reach the cutter with matrix attached. The advantage is geological certainty, since a stone taken from a known face has a documented source.

Ratnapura

Ratnapura, whose name means city of gems in Sinhala, is the historic centre of Sri Lanka's gem trade, where sapphire, chrysoberyl, spinel and garnet are won from alluvial gravels called illam.

Mining in the district remains largely small-scale: hand-sunk shafts propped with timber, gravel hauled up in baskets and washed in cane pans. Ratnapura material covers the full sapphire colour range including padparadscha, alongside star stones and chrysoberyl. Ministry of Gems has worked the Ceylon fields across three generations and mines its own sapphire ground there. Rough is usually traded locally before export, so documentation rarely follows a single stone from pit to counter.

Rubyvale

Rubyvale is a mining town in the Central Queensland gemfields which, despite its name, produces sapphire rather than ruby, recovered mainly from deep alluvial leads buried well below the surface.

The gem-bearing wash at Rubyvale sits in ancient buried stream channels, sometimes many metres down, so mining has depended on shafts and underground drives rather than surface stripping. Production covers the Queensland range of blue, green, golden yellow and parti-coloured sapphire, with occasional star stones. The name records red stones reported in the field's early days and remains a source of confusion for buyers, since gem-quality Australian ruby is very rare.

See also: Anakie · Alluvial deposit · Basalt field

Trade, Certification & Value

How stones are weighed, reported, priced and described.

AGL

AGL, American Gemological Laboratories in New York, is an independent laboratory specialising in coloured stones, issuing reports that assess colour, clarity, cut and finish alongside treatment and origin.

AGL is unusual in giving coloured stones a structured quality assessment rather than identification alone, which lets a buyer compare stones that sellers describe with the same loose adjectives. Its treatment reporting is granular, including the degree of clarity enhancement in emerald. The assessments remain expert opinion and set no price, so a stone should still be judged in the hand as well as on paper.

See also: Laboratory report · GIA · Trade term

Carat

The carat is the metric unit of gem weight, equal to exactly 0.2 grams or one-fifth of a gram, and it measures mass rather than a stone's physical dimensions.

Two stones of identical carat weight can look quite different in size, because denser species pack more mass into less volume; a one-carat sapphire measures smaller across the top than a one-carat diamond. Cut proportions alter apparent size again, so millimetre measurements matter as much as weight when a stone is worn. The carat is unrelated to the karat used for gold fineness.

See also: Point · Per-carat price · Size bracket

→ Sapphire price guide

Certificate

Certificate is the trade's everyday word for a gem laboratory report, though laboratories themselves avoid it, since their findings are a documented expert opinion on a single date rather than a guarantee.

Nothing in a so-called certificate certifies worth; it records identity, weight and treatment as assessed at one moment. Who issued it matters far more than the word on the header, and an unnamed or in-house laboratory carries much less weight than a recognised independent one. An insurance appraisal, which does state a replacement value, is a separate document written for a different purpose.

See also: Laboratory report · GIA · AGL

Chain of custody

Chain of custody is the documented, unbroken record of who has held and handled a gemstone at each stage, from recovery at the mine through cutting and setting to the final sale.

It is the practical evidence behind any sourcing claim, since parcels are routinely mixed and re-sorted in trading centres and, once mixed, origin cannot be reconstructed. Tagging schemes and blockchain records now exist for some material, while a house that mines and cuts its own stones can document custody directly. Custody records handling and sourcing only, and say nothing about a stone's quality or treatment.

See also: Provenance · Mine-to-market · Parcel

→ Gem Passport

CIBJO

CIBJO, the World Jewellery Confederation, is the international standards body whose Blue Books define the terminology and disclosure rules the trade uses for diamonds, coloured stones, pearls, precious metals and gem laboratories.

Its rules govern how the trade may describe what it sells: disclosure of treatment is mandatory under the Blue Books, and from September 2026 CIBJO retires the term laboratory-grown in favour of synthetic for man-made gems. The standards are voluntary in themselves but are adopted by national trade associations and cited by consumer regulators. They are why careful sellers use narrow, defined wording.

See also: Trade term · Laboratory report · Certificate

FGAA

FGAA stands for Fellow of the Gemmological Association of Australia, the post-nominal awarded to gemmologists who complete the association's Diploma in Gemmology and pass its practical stone-identification examinations.

In Australia, FGAA is the usual signal that the person identifying or grading a stone has been formally examined rather than trained by trade experience alone. It attests to an individual's competence, not to any particular stone, so on a significant purchase it complements rather than replaces an independent laboratory report. Ministry of Gems' managing director, Anna Lulinkova, is an FGAA gemmologist and grades every stone in Sydney.

See also: GAA · Laboratory report · Certificate

→ About our gemmologist

GAA

The Gemmological Association of Australia, abbreviated GAA, is the national body for gemmological education in Australia, delivering the Diploma in Gemmology and admitting its graduates as Fellows.

The association runs courses and examinations through state divisions and is the usual route to a formal gemmological qualification in Australia. For buyers it matters mainly as the source of the FGAA post-nominal seen after the names of Australian gemmologists. Note the distinction carefully: the association educates and examines people, and does not issue reports on individual gemstones, which is the work of a gem laboratory.

GIA

GIA, the Gemological Institute of America, is a non-profit gem laboratory and school founded in 1931 that devised the 4Cs and the D-to-Z colour scale now used for diamonds worldwide.

GIA reports are the most widely recognised in the diamond trade, and the institute also issues identification and treatment reports for coloured stones, adding geographic origin only on request and only for certain species. It does not give coloured stones an overall quality grade the way it grades diamonds, and it states no values. Its treatment-detection research is relied on across the industry.

See also: Laboratory report · Certificate · AGL

Gubelin

Gübelin Gem Lab, founded in Lucerne in 1923 and often written Gubelin, is a Swiss gem laboratory recognised for origin determination in coloured stones and for its research into inclusions and traceability.

The laboratory built much of the modern reference literature on inclusions, and its origin opinions are among the most quoted for Kashmir and Burmese material. It also developed traceability work, including nanoparticle tagging of emerald rough at the mine and a blockchain record opened to the wider trade. Because origin premiums rest on opinion, a differing second report can change what a stone is worth.

See also: SSEF · Provenance · Chain of custody

Laboratory report

A laboratory report is a document issued by an independent gem laboratory that records a stone's identity, weight, measurements and any detected treatment, stated as that laboratory's expert opinion on the date of testing.

A report is evidence of what a laboratory found, not a warranty of value, and terminology, thresholds and origin opinions vary between laboratories. Check that the weight and measurements on the paper match the stone in front of you, and note the issue date, since detection standards change over time. A report from a seller's own in-house laboratory is not the same thing as an independent one.

See also: Certificate · GIA · Provenance

Matched pair

A matched pair is two stones selected to agree closely in colour, tone, saturation, size, outline and cutting style, so that they read as identical when worn together in earrings.

Matching is exacting work, because a second stone must agree across every one of those axes at once, and a true pair can take years to assemble; that scarcity is why a pair usually costs more than two comparable single stones bought separately. Judge candidates face-up and side by side in indirect daylight. Identical weights matter less than visual balance once the stones are set.

See also: Parcel · Per-carat price · Size bracket

→ Matched sapphire pairs

Mine-to-market

Mine-to-market describes a vertically integrated business that controls its gems at every stage, from mining and sorting through cutting to final sale, instead of buying finished stones through intermediary dealers.

Integration matters to a buyer for two reasons: the seller can describe recovery and cutting from first-hand records, and fewer intermediary margins sit between the ground and the counter. It is not in itself a guarantee of better quality or a lower price, and it does not remove the need for treatment disclosure or independent testing on high-value stones. Ministry of Gems mines, cuts and grades its own Ceylon sapphire.

See also: Chain of custody · Provenance · Rough

Parcel

A parcel is a group of gemstones offered and priced as one lot, traditionally folded into a paper packet, with the buyer taking the whole group rather than selecting individual stones.

Most rough and most small calibrated goods move through the trade in parcels, priced per carat across the lot, so quality varies within it and the better stones effectively subsidise the weaker ones. Valuing that average is the dealer's skill. For a retail buyer, the fact that a stone came out of a parcel conveys nothing about its individual quality, which still needs assessing on its own merits.

See also: Per-carat price · Rough · Chain of custody

→ Trade & wholesale

Per-carat price

Per-carat price, the trade's standard unit of pricing, is the value agreed for one carat at a given quality, multiplied by a stone's weight to arrive at the total price.

Because larger crystals are rarer, per-carat price climbs as weight rises, so total cost increases faster than weight does; a two-carat stone of a given quality generally costs well more than twice its one-carat equivalent. Comparing offers by total price alone conceals this, and conceals differences in quality. Ask for the per-carat figure and the quality basis behind it before comparing stones.

See also: Carat · Size bracket · Parcel

→ Sapphire price guide

→ The Price of Colour

Pigeon's blood

Pigeon's blood is a laboratory colour opinion applied to the finest saturated red ruby, usually with strong red fluorescence; it describes appearance only and is neither an origin nor a guarantee of quality.

The major laboratories issue the term against their own criteria, and those criteria differ, so the same stone can carry the description on one report and not on another. Because it commands a substantial premium, it is worth reading exactly what the report says rather than trading on the phrase alone. 'Royal blue' is applied to sapphire on the same basis. Neither term is a grading standard, and neither is defined by any independent authority.

See also: Ruby · Royal blue · Laboratory report

Point

One point equals one-hundredth of a carat, or 0.002 grams, so a stone described in the trade as ninety points weighs 0.90 carats, just below the one-carat mark.

The trade quotes stones under a carat in points, and weight should always be recorded to two decimal places. The distinction matters most at the round figures, where a 0.90-carat and a 1.00-carat stone can be almost indistinguishable on the hand yet sit either side of a price step. Buyers willing to accept ninety-something points often gain on price for very little visible size.

See also: Carat · Size bracket · Per-carat price

Provenance

In gemmology, provenance covers both a stone's geographic origin and its documented history of ownership, and for celebrated deposits such as Kashmir, Mogok and Ceylon it can materially affect price.

Geographic origin is inferred by laboratories from inclusion scenes and trace-element chemistry rather than read from any label, and deposits with similar geology produce cautious or inconclusive calls. Reputable laboratories sometimes disagree on the same stone, so a premium paid for origin rests on an opinion that another laboratory may not share. Ownership provenance, such as a documented former collection, is assessed by archive as much as by gemmology.

See also: Chain of custody · SSEF · Gubelin

→ Gem Passport

Rough

Rough describes gem material in its natural, uncut state as it comes out of the ground, before any sawing, shaping or polishing has been carried out.

Cutting commonly removes more than half a crystal's weight, because the cutter must balance weight retention against correct proportions, so a well-proportioned finished stone usually began as a considerably larger piece. Rough is bought on judgement of what lies inside, since colour zoning, fractures and inclusions are only partly visible from the surface. It is a specialist trade rather than a shortcut to value for consumers.

See also: Parcel · Carat · Mine-to-market

Size bracket

Size brackets are the weight thresholds, commonly 1.00, 2.00, 3.00 and 5.00 carats, at which per-carat prices step upward, so crossing a round figure raises price disproportionately.

A 0.95-carat and a 1.05-carat stone can look the same once set, yet fall into different brackets and different price bands. A buyer who cares about how a stone looks rather than the figure printed on a report can often do better just below a threshold. The same pressure acts on cutters, who sometimes retain extra weight in the pavilion to reach a bracket, at some cost to proportion.

See also: Carat · Point · Per-carat price

→ The Price of Colour

SSEF

SSEF, the Swiss Gemmological Institute in Basel, is an independent laboratory known for testing high-value coloured stones and natural pearls and for its geographic-origin determinations.

SSEF reports carry particular weight at international auction, especially for Kashmir sapphire, Burmese ruby and natural pearls, and the laboratory combines microscopy with trace-element chemistry in its origin work. Colour appendices such as royal blue are the laboratory's colour opinions rather than grades, and other laboratories apply their own criteria. Two respected laboratories can reach different origin conclusions on the same stone.

See also: Gubelin · Provenance · Laboratory report