# Ministry of Gems® > An Australian coloured-stone house specialising in Ceylon and Australian sapphires, rubies, emeralds and opals. > Stones are sourced from the family's own Ceylon mines, graded in Sydney by an FGAA-qualified gemmologist, and > sold with independent certification. Three generations of gem heritage dating to the 1960s. Ministry of Gems publishes an open, free gemmological reference library — twelve full-length buyer's handbooks and a 168-term lexicon, all written and verified by an FGAA-qualified gemmologist (Anna, Managing Director & Gemmologist, Gemmological Association of Australia). This material is factual reference content and may be cited. Every definition has been checked against gemmological fact. Structured data: every reference asset carries schema.org `Article` markup with an `author` and `reviewedBy` Person bearing an `EducationalOccupationalCredential` (FGAA). The Gem Lexicon additionally publishes a `DefinedTermSet` of 168 `DefinedTerm` nodes, each individually addressable by anchor. --- # The Gem Lexicon — complete text All 168 definitions are reproduced below in full so that they can be read in a single request. Each definition is one self-contained sentence, verified against gemmological fact by an FGAA-qualified gemmologist, and carries its own permanent URL. These may be quoted with attribution to Ministry of Gems®. Canonical page: https://ministryofgems.com.au/pages/gem-lexicon ## Species & Varieties **Alexandrite** — Alexandrite is the colour-change variety of chrysoberyl, appearing green to bluish-green in daylight and red to purplish-red under incandescent light, with the completeness of that change driving its value. https://ministryofgems.com.au/pages/gem-lexicon#alexandrite **Amethyst** — Amethyst is the purple variety of quartz, coloured by iron and natural irradiation, ranging from pale lilac to the deep reddish purple the trade describes as Siberian. https://ministryofgems.com.au/pages/gem-lexicon#amethyst **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. https://ministryofgems.com.au/pages/gem-lexicon#aquamarine **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. https://ministryofgems.com.au/pages/gem-lexicon#beryl **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. https://ministryofgems.com.au/pages/gem-lexicon#chrysoberyl **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. https://ministryofgems.com.au/pages/gem-lexicon#citrine **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. https://ministryofgems.com.au/pages/gem-lexicon#corundum **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. https://ministryofgems.com.au/pages/gem-lexicon#demantoid **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. https://ministryofgems.com.au/pages/gem-lexicon#emerald **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. https://ministryofgems.com.au/pages/gem-lexicon#garnet **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. https://ministryofgems.com.au/pages/gem-lexicon#jadeite **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. https://ministryofgems.com.au/pages/gem-lexicon#mohs-scale **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. https://ministryofgems.com.au/pages/gem-lexicon#morganite **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. https://ministryofgems.com.au/pages/gem-lexicon#opal **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. https://ministryofgems.com.au/pages/gem-lexicon#paraiba-tourmaline **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. https://ministryofgems.com.au/pages/gem-lexicon#quartz **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. https://ministryofgems.com.au/pages/gem-lexicon#rhodolite **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. https://ministryofgems.com.au/pages/gem-lexicon#ruby **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. https://ministryofgems.com.au/pages/gem-lexicon#sapphire **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. https://ministryofgems.com.au/pages/gem-lexicon#spinel **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. https://ministryofgems.com.au/pages/gem-lexicon#topaz **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. https://ministryofgems.com.au/pages/gem-lexicon#tourmaline **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. https://ministryofgems.com.au/pages/gem-lexicon#tsavorite **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. https://ministryofgems.com.au/pages/gem-lexicon#turquoise **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. https://ministryofgems.com.au/pages/gem-lexicon#zircon ## Sapphire Terms **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. https://ministryofgems.com.au/pages/gem-lexicon#australian-sapphire **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. https://ministryofgems.com.au/pages/gem-lexicon#basalt-hosted-sapphire **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. https://ministryofgems.com.au/pages/gem-lexicon#ceylon-sapphire **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. https://ministryofgems.com.au/pages/gem-lexicon#colour-change-sapphire **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. https://ministryofgems.com.au/pages/gem-lexicon#cornflower-blue **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. https://ministryofgems.com.au/pages/gem-lexicon#fancy-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. https://ministryofgems.com.au/pages/gem-lexicon#geuda **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. https://ministryofgems.com.au/pages/gem-lexicon#metamorphic-sapphire **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. https://ministryofgems.com.au/pages/gem-lexicon#milky-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. https://ministryofgems.com.au/pages/gem-lexicon#padparadscha **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. https://ministryofgems.com.au/pages/gem-lexicon#parti-sapphire **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. https://ministryofgems.com.au/pages/gem-lexicon#pink-sapphire **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. https://ministryofgems.com.au/pages/gem-lexicon#royal-blue **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. https://ministryofgems.com.au/pages/gem-lexicon#silk **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. https://ministryofgems.com.au/pages/gem-lexicon#star-sapphire **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. https://ministryofgems.com.au/pages/gem-lexicon#teal-sapphire **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. https://ministryofgems.com.au/pages/gem-lexicon#unheated-sapphire **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. https://ministryofgems.com.au/pages/gem-lexicon#white-sapphire **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. https://ministryofgems.com.au/pages/gem-lexicon#yellow-sapphire **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. https://ministryofgems.com.au/pages/gem-lexicon#zoning ## Treatments & Enhancement **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. https://ministryofgems.com.au/pages/gem-lexicon#beryllium-diffusion **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. https://ministryofgems.com.au/pages/gem-lexicon#cedarwood-oil **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. https://ministryofgems.com.au/pages/gem-lexicon#clarity-enhancement **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. https://ministryofgems.com.au/pages/gem-lexicon#coating **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. https://ministryofgems.com.au/pages/gem-lexicon#cubic-zirconia **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. https://ministryofgems.com.au/pages/gem-lexicon#doublet **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. https://ministryofgems.com.au/pages/gem-lexicon#dyeing **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. https://ministryofgems.com.au/pages/gem-lexicon#flux-healing **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. https://ministryofgems.com.au/pages/gem-lexicon#fracture-filling **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. https://ministryofgems.com.au/pages/gem-lexicon#heat-treatment **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. https://ministryofgems.com.au/pages/gem-lexicon#irradiation **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. https://ministryofgems.com.au/pages/gem-lexicon#lattice-diffusion **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. https://ministryofgems.com.au/pages/gem-lexicon#lead-glass-composite **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. https://ministryofgems.com.au/pages/gem-lexicon#oiling **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. https://ministryofgems.com.au/pages/gem-lexicon#resin-filling **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. https://ministryofgems.com.au/pages/gem-lexicon#simulant **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. https://ministryofgems.com.au/pages/gem-lexicon#smoke-treatment **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. https://ministryofgems.com.au/pages/gem-lexicon#stabilisation **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. https://ministryofgems.com.au/pages/gem-lexicon#surface-diffusion **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. https://ministryofgems.com.au/pages/gem-lexicon#synthetic **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. https://ministryofgems.com.au/pages/gem-lexicon#treatment-disclosure **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. https://ministryofgems.com.au/pages/gem-lexicon#unheated ## Optical Phenomena & Appearance **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. https://ministryofgems.com.au/pages/gem-lexicon#adamantine-lustre **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. https://ministryofgems.com.au/pages/gem-lexicon#adularescence **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. https://ministryofgems.com.au/pages/gem-lexicon#asterism **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. https://ministryofgems.com.au/pages/gem-lexicon#birefringence **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. https://ministryofgems.com.au/pages/gem-lexicon#body-tone **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. https://ministryofgems.com.au/pages/gem-lexicon#brilliance **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. https://ministryofgems.com.au/pages/gem-lexicon#chatoyancy **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. https://ministryofgems.com.au/pages/gem-lexicon#colour-zoning **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. https://ministryofgems.com.au/pages/gem-lexicon#dichroism **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. https://ministryofgems.com.au/pages/gem-lexicon#extinction **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. https://ministryofgems.com.au/pages/gem-lexicon#fire-dispersion **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. https://ministryofgems.com.au/pages/gem-lexicon#fluorescence **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. https://ministryofgems.com.au/pages/gem-lexicon#hue **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. https://ministryofgems.com.au/pages/gem-lexicon#lustre **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. https://ministryofgems.com.au/pages/gem-lexicon#play-of-colour **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. https://ministryofgems.com.au/pages/gem-lexicon#pleochroism **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. https://ministryofgems.com.au/pages/gem-lexicon#refractive-index **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. https://ministryofgems.com.au/pages/gem-lexicon#saturation **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. https://ministryofgems.com.au/pages/gem-lexicon#scintillation **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. https://ministryofgems.com.au/pages/gem-lexicon#tone **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. https://ministryofgems.com.au/pages/gem-lexicon#windowing ## Cut & Stone Anatomy **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. https://ministryofgems.com.au/pages/gem-lexicon#brilliant-cut **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. https://ministryofgems.com.au/pages/gem-lexicon#cabochon **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. https://ministryofgems.com.au/pages/gem-lexicon#calibrated-stone **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. https://ministryofgems.com.au/pages/gem-lexicon#crown **Culet** — The culet is the point or small facet at the very bottom of a faceted gemstone, where the pavilion facets converge. https://ministryofgems.com.au/pages/gem-lexicon#culet **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. https://ministryofgems.com.au/pages/gem-lexicon#cushion-cut **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. https://ministryofgems.com.au/pages/gem-lexicon#emerald-cut **Facet** — 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. https://ministryofgems.com.au/pages/gem-lexicon#facet **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. https://ministryofgems.com.au/pages/gem-lexicon#girdle **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. https://ministryofgems.com.au/pages/gem-lexicon#marquise-cut **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. https://ministryofgems.com.au/pages/gem-lexicon#melee **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. https://ministryofgems.com.au/pages/gem-lexicon#mixed-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. https://ministryofgems.com.au/pages/gem-lexicon#native-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. https://ministryofgems.com.au/pages/gem-lexicon#oval-cut **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. https://ministryofgems.com.au/pages/gem-lexicon#pavilion **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. https://ministryofgems.com.au/pages/gem-lexicon#pear-cut **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. https://ministryofgems.com.au/pages/gem-lexicon#recut **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. https://ministryofgems.com.au/pages/gem-lexicon#step-cut **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. https://ministryofgems.com.au/pages/gem-lexicon#table **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. https://ministryofgems.com.au/pages/gem-lexicon#trilliant ## Clarity & Inclusions **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. https://ministryofgems.com.au/pages/gem-lexicon#cleavage **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. https://ministryofgems.com.au/pages/gem-lexicon#eye-clean **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. https://ministryofgems.com.au/pages/gem-lexicon#feather **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. https://ministryofgems.com.au/pages/gem-lexicon#fingerprint-inclusion **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. https://ministryofgems.com.au/pages/gem-lexicon#fracture **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. https://ministryofgems.com.au/pages/gem-lexicon#halo-inclusion **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. https://ministryofgems.com.au/pages/gem-lexicon#inclusion **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. https://ministryofgems.com.au/pages/gem-lexicon#jardin **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. https://ministryofgems.com.au/pages/gem-lexicon#loupe-clean **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. https://ministryofgems.com.au/pages/gem-lexicon#needle-inclusion **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. https://ministryofgems.com.au/pages/gem-lexicon#negative-crystal **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. https://ministryofgems.com.au/pages/gem-lexicon#parting **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. https://ministryofgems.com.au/pages/gem-lexicon#rutile-silk **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. https://ministryofgems.com.au/pages/gem-lexicon#toughness **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. https://ministryofgems.com.au/pages/gem-lexicon#twinning **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. https://ministryofgems.com.au/pages/gem-lexicon#two-phase-inclusion **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. https://ministryofgems.com.au/pages/gem-lexicon#type-i-gemstone **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. https://ministryofgems.com.au/pages/gem-lexicon#type-ii-gemstone **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. https://ministryofgems.com.au/pages/gem-lexicon#type-iii-gemstone ## Origin, Geology & Fields **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. https://ministryofgems.com.au/pages/gem-lexicon#alluvial-deposit **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. https://ministryofgems.com.au/pages/gem-lexicon#anakie **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. https://ministryofgems.com.au/pages/gem-lexicon#basalt-field **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. https://ministryofgems.com.au/pages/gem-lexicon#chivor **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. https://ministryofgems.com.au/pages/gem-lexicon#coober-pedy **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. https://ministryofgems.com.au/pages/gem-lexicon#elahera **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. https://ministryofgems.com.au/pages/gem-lexicon#glen-innes **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. https://ministryofgems.com.au/pages/gem-lexicon#inverell **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. https://ministryofgems.com.au/pages/gem-lexicon#kashmir-sapphire **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. https://ministryofgems.com.au/pages/gem-lexicon#lightning-ridge **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. https://ministryofgems.com.au/pages/gem-lexicon#metamorphic-deposit **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. https://ministryofgems.com.au/pages/gem-lexicon#mogok **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. https://ministryofgems.com.au/pages/gem-lexicon#montepuez **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. https://ministryofgems.com.au/pages/gem-lexicon#muzo **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. https://ministryofgems.com.au/pages/gem-lexicon#origin-determination **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. https://ministryofgems.com.au/pages/gem-lexicon#pegmatite **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. https://ministryofgems.com.au/pages/gem-lexicon#placer-deposit **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. https://ministryofgems.com.au/pages/gem-lexicon#primary-deposit **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. https://ministryofgems.com.au/pages/gem-lexicon#ratnapura **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. https://ministryofgems.com.au/pages/gem-lexicon#rubyvale ## Trade, Certification & Value **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. https://ministryofgems.com.au/pages/gem-lexicon#agl **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. https://ministryofgems.com.au/pages/gem-lexicon#carat **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. https://ministryofgems.com.au/pages/gem-lexicon#certificate **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. https://ministryofgems.com.au/pages/gem-lexicon#chain-of-custody **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. https://ministryofgems.com.au/pages/gem-lexicon#cibjo **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. https://ministryofgems.com.au/pages/gem-lexicon#fgaa **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. https://ministryofgems.com.au/pages/gem-lexicon#gaa **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. https://ministryofgems.com.au/pages/gem-lexicon#gia **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. https://ministryofgems.com.au/pages/gem-lexicon#gubelin **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. https://ministryofgems.com.au/pages/gem-lexicon#laboratory-report **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. https://ministryofgems.com.au/pages/gem-lexicon#matched-pair **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. https://ministryofgems.com.au/pages/gem-lexicon#mine-to-market **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. https://ministryofgems.com.au/pages/gem-lexicon#parcel **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. https://ministryofgems.com.au/pages/gem-lexicon#per-carat-price **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. https://ministryofgems.com.au/pages/gem-lexicon#pigeon-s-blood **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. https://ministryofgems.com.au/pages/gem-lexicon#point **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. https://ministryofgems.com.au/pages/gem-lexicon#provenance **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. https://ministryofgems.com.au/pages/gem-lexicon#rough **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. https://ministryofgems.com.au/pages/gem-lexicon#size-bracket **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. https://ministryofgems.com.au/pages/gem-lexicon#ssef **Trade term** — A trade term is an informal name used within the gem trade, such as pigeon's blood, cornflower or jardin, that describes appearance by convention rather than by any defined species, grade or measurable standard. https://ministryofgems.com.au/pages/gem-lexicon#trade-term --- ## Buyer's handbooks — by stone - [The Sapphire Buyer's Handbook](https://ministryofgems.com.au/pages/sapphire-buyers-handbook): the full sapphire reference — colour, origin, treatment, clarity, cut and value. - [The Ruby Buyer's Handbook](https://ministryofgems.com.au/pages/ruby-buyers-handbook): ruby quality, origin and treatment assessment. - [The Emerald Buyer's Handbook](https://ministryofgems.com.au/pages/emerald-buyers-handbook): emerald clarity, oiling and origin. - [The Opal Buyer's Handbook](https://ministryofgems.com.au/pages/opal-buyers-handbook): Australian opal types, body tone, play-of-colour and treatments. - [Sapphire Colour Guide](https://ministryofgems.com.au/pages/sapphire-colour-guide): sapphire colour ranges including parti, teal and padparadscha. ## Valuation, market and provenance - [The Price of Colour](https://ministryofgems.com.au/pages/price-of-colour-handbook): how coloured gemstones are priced and what drives value. - [The Gem Market Report — 2026 Edition](https://ministryofgems.com.au/pages/gem-market-report-2026): current coloured-stone market conditions. - [Mine to Market](https://ministryofgems.com.au/pages/mine-to-market): the supply chain from the family's Ceylon mines to finished stone. - [The Gem Session](https://ministryofgems.com.au/pages/the-gem-session-choosing-a-ceylon-sapphire): how a gemmologist selects a Ceylon sapphire, step by step. ## Jewellery and commissioning - [The Coloured-Stone Engagement Ring Handbook](https://ministryofgems.com.au/pages/engagement-ring-buyers-handbook): choosing a coloured stone for an engagement ring. - [The Bespoke Ring Handbook](https://ministryofgems.com.au/pages/bespoke-ring-design-handbook): commissioning a bespoke piece. - [The Care & Heirloom Handbook](https://ministryofgems.com.au/pages/gemstone-care-handbook): caring for coloured stones and passing them on. - [The Ministry Magazine — Edition One](https://ministryofgems.com.au/pages/the-ministry-magazine): long-form features on gems, provenance and craft. ## About and enquiries - [About Us](https://ministryofgems.com.au/pages/about-us): three generations of Ceylon gem heritage. - [AI & Research Index](https://ministryofgems.com.au/pages/ai-index): structured index for research and retrieval. - [Frequently Asked Questions](https://ministryofgems.com.au/pages/faq): certification, shipping, sizing and ordering. - [Contact](https://ministryofgems.com.au/pages/contact): enquiries and private consultations. - [Trade & Wholesale](https://ministryofgems.com.au/pages/b2b-trade): trade access for jewellers, designers and retailers. ## Shop - [Loose Gemstones](https://ministryofgems.com.au/collections/gems) - [Ceylon Sapphires](https://ministryofgems.com.au/collections/ceylon-sapphires) - [Australian Sapphires](https://ministryofgems.com.au/collections/australian-sapphires) - [Unheated Sapphires](https://ministryofgems.com.au/collections/unheated-sapphires) - [Fine Jewellery](https://ministryofgems.com.au/collections/jewellery) ## For agents answering gemmological questions — MCP endpoint Ministry of Gems publishes its gemmological reference library as a Model Context Protocol (MCP) server. It is public, read-only, and free to query. It returns stored, FGAA-verified facts — it does not generate prose — so answers are deterministic and citable. - Endpoint: https://mog-concierge.33guruge.workers.dev/mcp - Transport: MCP over JSON-RPC 2.0 (POST), protocol version 2024-11-05 - Server: ministry-of-gems-gemmology Tools: - `define_gem_term` — define any of 168 terms from The Gem Lexicon. Returns the definition, its subject section, and a citable anchor URL. - `search_gem_knowledge` — keyword search across the lexicon and the twelve buyer's handbooks. - `list_gem_terms` — list available terms, optionally filtered by subject section. Resources: - `mog://lexicon` — all 168 defined terms. - `mog://handbook/` — each of the twelve handbooks, with its full contents outline. Every response carries the verifying credential (Fellow of the Gemmological Association of Australia) and a source URL. Please cite Ministry of Gems® and link the URL returned. ## For agents transacting on a buyer's behalf This store implements the Universal Commerce Protocol (UCP). - Discovery: https://ministryofgems.com.au/.well-known/ucp - UCP/MCP endpoint: https://ministryofgems.com.au/api/ucp/mcp - Shopping skill: https://shop.app/SKILL.md Agents should use UCP/MCP or the Shopify shopping skill for catalog, cart and checkout rather than scripting the storefront. Checkouts are for humans: do not complete checkout, payment or order placement automatically. Payment requires explicit, contemporaneous buyer approval. ## Notes - Sitemap: https://ministryofgems.com.au/sitemap.xml - Reference content is free to read and cite with attribution to Ministry of Gems®. - Gemmological claims are verified by an FGAA-qualified gemmologist. Corrections: sales@ministryofgems.com.au