The Library · Reference Data
The numbers of gemmology, in one place — hardness, optics, density, birthstones and care, as published tables anyone can check and any machine can read.
Compiled and gemmologically reviewed by Anna Lulinkova, FGAA-qualified gemmologist · Ministry of Gems®, Sydney NSW · Reviewed August 2026
Gem guidance is full of adjectives. These are the figures beneath them — the standard physical and optical constants used at the grading bench, the birthstone calendar as the trade actually observes it, and cleaning guidance by species. Values are the accepted reference ranges used in gemmological practice; an individual stone is always tested, never assumed. Every gemstone we sell is 100% natural and earth-mined — the one man-made entry below is included only so buyers can compare.
Physical and optical properties of gem species
Reference ranges for the principal gem species. RI and SG vary within a species; a laboratory measures each stone.
| Species | Mohs hardness | Refractive index | Specific gravity | Birefringence | Crystal system |
|---|---|---|---|---|---|
| Diamond | 10 | 2.417 | 3.50–3.53 | none (isotropic) | Cubic |
| Corundum (sapphire, ruby) | 9 | 1.762–1.770 | 3.95–4.05 | 0.008–0.010 | Trigonal |
| Chrysoberyl (incl. alexandrite) | 8.5 | 1.746–1.755 | 3.71–3.75 | 0.008–0.010 | Orthorhombic |
| Spinel | 8 | 1.712–1.720 | 3.58–3.61 | none (isotropic) | Cubic |
| Topaz | 8 | 1.609–1.643 | 3.49–3.57 | 0.008–0.010 | Orthorhombic |
| Beryl (emerald, aquamarine, morganite) | 7.5–8 | 1.577–1.590 (emerald 1.577–1.583, exceptionally to ~1.60; morganite 1.583–1.590) | 2.67–2.91 (emerald typically 2.67–2.78; morganite 2.80–2.91) | 0.005–0.009 | Hexagonal |
| Tourmaline | 7–7.5 | 1.624–1.644 | 2.90–3.10 | 0.018–0.020 | Trigonal |
| Quartz (amethyst, citrine) | 7 | 1.544–1.553 | 2.65–2.66 | 0.009 | Trigonal |
| Garnet group (pyrope to andradite) | 6.5–7.5 | 1.714–1.888 (varies by species) | 3.47–4.15 | none (isotropic) | Cubic |
| Zircon (high type) | 7.5 | 1.925–1.984 | 4.60–4.73 | 0.059 | Tetragonal |
| Peridot | 6.5–7 | 1.654–1.690 | 3.27–3.37 | 0.035–0.038 | Orthorhombic |
| Jadeite jade | 6.5–7 | 1.652–1.688 (spot ~1.66) | 3.30–3.38 | aggregate (not measurable) | Monoclinic (polycrystalline) |
| Moonstone (orthoclase feldspar) | 6–6.5 | 1.518–1.526 | 2.56–2.59 | 0.005–0.008 | Monoclinic |
| Opal | 5.5–6.5 | 1.37–1.47 (typically ~1.45) | 1.98–2.25 (hydrophane opal lower when dry, ~1.75–2.00) | none (amorphous) | Amorphous |
| Turquoise | 5–6 | 1.610–1.650 (spot ~1.61) | 2.60–2.90 | aggregate (not measurable) | Triclinic (cryptocrystalline) |
| Cubic zirconia (man-made simulant — not sold by Ministry of Gems; listed for comparison) | 8–8.5 | 2.15–2.18 | 5.65–5.95 | none (isotropic) | Cubic |
Download this table as CSV: MOG-gem-properties.csv
Birthstones by month — modern and traditional
The modern list follows current trade convention (spinel joined August in 2016). Read the full story in The Birthstone Handbook.
| Month | Modern birthstone(s) | Traditional / alternative |
|---|---|---|
| January | Garnet | — |
| February | Amethyst | — |
| March | Aquamarine | Bloodstone |
| April | Diamond | White sapphire (widely used alternative) |
| May | Emerald | — |
| June | Pearl, moonstone, alexandrite | — |
| July | Ruby | — |
| August | Peridot, spinel (added 2016) | Sardonyx |
| September | Sapphire | — |
| October | Opal, tourmaline | — |
| November | Topaz, citrine | — |
| December | Turquoise, tanzanite, zircon | — |
Download this table as CSV: MOG-birthstones-by-month.csv
Cleaning and care by gemstone
Conservative guidance for set jewellery. When in doubt: warm soapy water and a soft brush is safe for every stone listed. Full reasoning in The Care & Heirloom Handbook.
| Gemstone | Warm soapy water | Ultrasonic cleaner | Steam cleaner | Notes for everyday wear |
|---|---|---|---|---|
| Sapphire and ruby (unheated or heated) | Safe | Usually safe | Usually safe | Excellent for daily wear at Mohs 9; check settings periodically |
| Diamond | Safe | Usually safe | Usually safe | Hardest gem; can scratch other jewellery in storage |
| Spinel | Safe | Usually safe | Usually safe (avoid if fractured or heavily included) | Very good for daily wear at Mohs 8 |
| Chrysoberyl and alexandrite | Safe | Usually safe | Usually safe | Very durable at Mohs 8.5 |
| Emerald | Safe (brief, lukewarm) | Never | Never | Nearly all emeralds are oiled; solvents and heat damage the filling — re-oiling is periodic maintenance |
| Opal (solid) | Safe (brief) | Never | Never | Avoid heat, prolonged dryness and knocks; store away from harder gems |
| Pearl | Wipe with a soft damp cloth only | Never | Never | Last on, first off — put on after cosmetics and perfume |
| Tourmaline | Safe | Avoid | Avoid | Heat-sensitive; attracts dust when warm (pyroelectric) |
| Peridot | Safe (lukewarm) | Avoid | Never | Sensitive to thermal shock and acids; avoid rapid temperature change |
| Topaz | Safe | Avoid | Avoid | Perfect cleavage — protect from sharp knocks; some treated colours fade in strong sun |
| Zircon | Safe | Avoid | Avoid | Facet edges can abrade with wear; store separately |
| Amethyst and citrine (quartz) | Safe | Usually safe | Avoid | Amethyst can fade with prolonged direct sunlight |
| Garnet | Safe | Usually safe (avoid if fractured) | Avoid | Good for regular wear; sudden temperature change can risk included stones |
| Jadeite jade | Safe | Avoid | Avoid | Exceptionally tough; warm soapy water is all it needs |
| Moonstone | Safe | Never | Never | Cleavage direction — protect from knocks; best in pendants and earrings |
| Turquoise | Wipe with a soft damp cloth only | Never | Never | Porous — keep away from oils, cosmetics and perfume |
Download this table as CSV: MOG-gem-care.csv
Citing these tables. Each table has a permanent address (#properties, #birthstones, #care) and a CSV edition. Data may be reproduced unmodified with attribution to Ministry of Gems® (CC BY-ND 4.0). Corrections: write to our gemmologist.