RI, SG, Mohs hardness, optic character, and the key diagnostic feature for 105 stones. Print it or save as PDF.
Other cheat sheets: All 130 stones · Gem-A FGA / FEEG
| Gem | RI | SG | Mohs | Optic | Key diagnostic feature |
|---|---|---|---|---|---|
| Agate | 1.530–1.543 | 2.58–2.64 | 6.5–7.0 | AGG | Characteristic concentric or parallel banding in multiple colors — primary visual identifier |
| Alexandrite | 1.746–1.755 | 3.70–3.78 | 9 | B+ | Dramatic color change |
| Almandine Garnet | 1.770–1.830 | 3.93–4.30 | 7.0–7.5 | SR | Deep red color |
| Amazonite | 1.514–1.530 | 2.54–2.58 | 6.0–6.5 | B− | Whitish cross-hatch twinning (perthitic grid) visible under magnification — diagnostic |
| Amethyst | 1.544–1.553 | 2.63–2.65 | 7 | U+ | 'Bull's eye' optic figure |
| Ametrine | 1.544–1.553 | 2.63–2.67 | 7 | U+ | Distinct bicolor zones — purple amethyst and yellow/orange citrine in same crystal |
| Andalusite | 1.629–1.649 | 3.13–3.21 | 7.0–7.5 | B− | Striking trichroism: green / red-brown / yellow-orange, often visible to naked eye |
| Apatite | 1.628–1.649 | 3.17–3.23 | 5 | U− | Hardness exactly 5.0 (Mohs reference mineral) — scratched by a knife or steel file |
| Aquamarine | 1.577–1.583 | 2.68–2.78 | 7.5–8.0 | U− | Low RI and SG |
| Aventurine | 1.544–1.553 | 2.64–2.69 | 7 | AGG | Aventurescence from flat fuchsite (green mica) platelets — visible under magnification |
| Benitoite | 1.757–1.804 | 3.64–3.68 | 6.0–6.5 | U+ | Vivid strong blue-white SW UV fluorescence — near-unique diagnostic property |
| Black Opal | 1.440–1.460 | 2.09–2.16 | 5.5–6.5 | SR | Dark body tone (N1–N4 on GIA tone scale) with vivid play-of-color |
| Black Spinel | 1.712–1.730 | 3.58–3.61 | 8 | SR | Singly refractive |
| Bloodstone | 1.530–1.543 | 2.58–2.64 | 6.5–7.0 | AGG | Dark green opaque chalcedony with distinctive red/orange spots — diagnostically useful visual appearance |
| Blue Diamond | 2.417 | 3.50–3.53 | 10 | SR | Strong blue phosphorescence after SW UV (5–30+ seconds) |
| Brown Diamond | 2.417 | 3.51–3.53 | 10 | SR | Most common fancy color — approximately 80% of all diamonds are brown or near-colorless |
| Carnelian | 1.530–1.543 | 2.58–2.64 | 6.5–7.0 | AGG | Translucent orange-red to red-brown body color — primary identifier |
| Cassiterite | 2.010–2.100 | 6.80–7.10 | 6.0–7.0 | U+ | Extremely high specific gravity |
| Cat's Eye Chrysoberyl | 1.746–1.755 | 3.70–3.78 | 9 | B+ | 'Milk and honey' effect: tilting the stone under a single point light reveals one half whitish ('milk') and one half gold/yellow ('honey') — unique to chrysoberyl among chatoyant gems |
| Chalcedony | 1.530–1.543 | 2.58–2.64 | 6.5–7.0 | AGG | Microcrystalline quartz aggregate — RI ~1.535 lower than macrocrystalline quartz (1.544–1.553) |
| Chrome Diopside | 1.664–1.730 | 3.22–3.38 | 5.5–6.5 | B+ | Chelsea filter red — chromium coloring (confirms Cr; does not distinguish from emerald without RI) |
| Chrome Tourmaline | 1.624–1.660 | 3.00–3.20 | 7.0–7.5 | U− | Chelsea filter RED — chromium coloring causes strong red reaction, unlike Fe-colored verdelite |
| Chrysoberyl | 1.746–1.755 | 3.70–3.78 | 9 | B+ | High hardness of 8.5 |
| Chrysoprase | 1.530–1.543 | 2.58–2.64 | 6.5–7.0 | AGG | Apple-green to yellow-green translucent body color caused by nickel — primary identifier |
| Citrine | 1.544–1.553 | 2.63–2.65 | 7 | U+ | 'Bull's eye' optic figure |
| Color-Change Garnet | 1.740–1.765 | 3.65–3.80 | 7.0–7.5 | SR | Singly refractive (isotropic) — stays dark under polariscope; alexandrite is doubly refractive and blinks |
| Color-Change Sapphire | 1.762–1.770 | 3.99–4.01 | 9 | U− | RI 1.762 — higher than alexandrite (1.746) — primary physical separator |
| Danburite | 1.627–1.636 | 2.97–3.02 | 7.0–7.5 | B− | Strong sky-blue LW and SW fluorescence — highly distinctive |
| Demantoid Garnet | 1.880–1.895 | 3.82–3.88 | 6.5–7.0 | SR | Dispersion higher than diamond |
| Diamond | 2.417 | 3.50–3.53 | 10 | SR | Adamantine luster |
| Emerald | 1.565–1.602 | 2.67–2.78 | 7.5–8.0 | U− | Shows red through Chelsea filter |
| Enstatite | 1.663–1.673 | 3.20–3.30 | 5.0–5.5 | B+ | Chelsea filter red reaction — chromium-bearing; shared with chrome diopside but different RI/SG |
| Fire Opal | 1.370–1.470 | 1.98–2.20 | 5.5–6.5 | SR | Orange-red body color with no play-of-color (typically) |
| Fluorite | 1.432–1.436 | 3.17–3.19 | 4 | SR | Low hardness (4) |
| Goshenite | 1.577–1.583 | 2.68–2.80 | 7.5–8.0 | U− | RI 1.577–1.583 clearly lower than white sapphire (1.762) and white topaz (1.619–1.627) |
| Green Tourmaline | 1.616–1.650 | 3.00–3.10 | 7.0–7.5 | U− | Much lower RI (1.624-1.644) than green sapphire (1.762) or tsavorite (1.740) |
| Grossular Garnet | 1.734–1.759 | 3.57–3.73 | 6.5–7.5 | SR | Colorless (leuco) to delicate mint green — paler, softer green than tsavorite (which is rich saturated green) |
| Hawk's Eye | 1.544–1.553 | 2.64–2.71 | 7 | AGG | Blue-grey to blue-green color from unoxidized crocidolite (unlike golden Tiger's Eye) |
| Heliodor | 1.568–1.580 | 2.68–2.74 | 7.5–8.0 | U− | Beryl RI and SG — separates from all yellow simulants |
| Hessonite Garnet | 1.730–1.760 | 3.59–3.65 | 7.0–7.5 | SR | Treacly or roiled internal appearance |
| Hiddenite | 1.660–1.676 | 3.15–3.21 | 6.5–7.0 | B+ | Perfect cleavage in two directions — chips easily along cleavage planes |
| Indicolite | 1.624–1.644 | 3.02–3.10 | 7.0–7.5 | U− | Strong dichroism — dark blue vs. light blue/colorless when rotated |
| Iolite | 1.535–1.547 | 2.57–2.66 | 7.0–7.5 | B− | Extreme pleochroism |
| Jadeite | 1.654–1.688 | 3.24–3.43 | 6.5–7.0 | AGG | Interlocking blocky granular texture (orange-peel surface on polished stones) |
| Kornerupine | 1.660–1.699 | 3.27–3.45 | 7 | B− | Strong trichroism (colorless / brown-yellow / green) — visible to naked eye in some specimens |
| Kunzite | 1.660–1.676 | 3.15–3.21 | 6.5–7.0 | B+ | Strong pleochroism |
| Kyanite | 1.712–1.734 | 3.53–3.68 | 4.5–7.0 | B− | Variable hardness by direction — 4.5 along crystal length, 6.5–7.0 across (unique among gemstones) |
| Labradorite | 1.559–1.573 | 2.68–2.75 | 6.0–6.5 | B− | Labradorescence |
| Lapis Lazuli | 1.500–1.670 | 2.70–2.90 | 5.0–6.0 | AGG | Intense blue color |
| Low-Type Zircon | 1.780–1.815 | 3.90–4.10 | 6.0–7.5 | SR | Metamict state (damaged crystal lattice) |
| Malachite | 1.655–1.909 | 3.60–4.05 | 3.5–4.0 | B− | Concentric banded green patterns — swirling bull's-eye and wavy bands are the primary visual identifier |
| Malaya / Umbalite Garnet | 1.730–1.780 | 3.65–3.87 | 7.0–7.5 | SR | Peachy orange-pink color unusual in the gem world — distinct from orange sapphire and padparadscha |
| Moldavite | 1.480–1.510 | 2.32–2.38 | 6 | SR | Unique sculptured/etched exterior surface with flow ridges, pitting, and corrosion features |
| Moonstone | 1.518–1.526 | 2.56–2.59 | 6.0–6.5 | B− | Adularescence (schiller) |
| Morganite | 1.572–1.600 | 2.71–2.85 | 7.5–8.0 | U− | Higher RI and SG than other beryls |
| Nephrite | 1.600–1.627 | 2.90–3.03 | 6.0–6.5 | AGG | Fibrous, felted texture |
| Obsidian | 1.480–1.510 | 2.30–2.60 | 5.0–5.5 | SR | Amorphous volcanic glass |
| Onyx | 1.530–1.543 | 2.58–2.64 | 6.5–7.0 | AGG | Opaque black body color — most common form in trade |
| Opal | 1.370–1.470 | 1.98–2.25 | 5.5–6.5 | SR | Play-of-color |
| Padparadscha Sapphire | 1.762–1.770 | 3.95–4.03 | 9 | U− | Unique pinkish-orange to orangey-pink hue — named 'lotus flower' color |
| Paraiba Tourmaline | 1.624–1.644 | 3.02–3.10 | 7.0–7.5 | U− | Electric neon blue-green — unmistakable when genuine |
| Peridot | 1.654–1.690 | 3.27–3.48 | 6.5–7.0 | B+ | Very strong facet doubling |
| Phenakite | 1.654–1.670 | 2.95–2.97 | 7.5–8.0 | U+ | Higher RI than quartz (1.544) and higher than white topaz (1.619–1.627) — phenakite RI 1.654–1.670 exceeds topaz |
| Pink Diamond | 2.417 | 3.51–3.53 | 10 | SR | Single refractive index 2.417 — highest of any natural gemstone |
| Pink Sapphire | 1.762–1.770 | 3.98–4.01 | 9 | U− | Strong red LW fluorescence (especially Myanmar material) — stronger than rubellite |
| Prasiolite | 1.544–1.553 | 2.63–2.67 | 7 | U+ | Pale leek-green to mint green color distinct from deeper greens of tourmaline or peridot |
| Prehnite | 1.611–1.665 | 2.82–2.94 | 6.0–6.5 | B+ | Pale apple-green to yellow-green color typical |
| Pyrope Garnet | 1.730–1.760 | 3.62–3.87 | 7.0–7.5 | SR | Blood-red color |
| Rainbow Moonstone | 1.559–1.572 | 2.68–2.72 | 6.0–6.5 | B− | RI 1.559–1.572 — ABOVE true moonstone (1.518–1.526); this single test identifies it |
| Red Beryl | 1.564–1.574 | 2.66–2.70 | 7.5–8.0 | U− | Extremely rare — essentially restricted to Wah Wah Mountains, Utah; faceted stones over 1ct are exceptional |
| Rhodochrosite | 1.600–1.820 | 3.40–3.70 | 3.5–4.0 | U− | Extreme birefringence 0.220 — back facets visibly doubled even to the naked eye |
| Rhodolite Garnet | 1.740–1.770 | 3.74–3.94 | 7.0–7.5 | SR | Distinctive purplish-red to raspberry red color |
| Rhodonite | 1.716–1.752 | 3.57–3.76 | 5.5–6.5 | B+ | Pink to red color |
| Rock Crystal | 1.544–1.553 | 2.63–2.65 | 7 | U+ | 'Bull's eye' optic figure |
| Rose Quartz | 1.544–1.553 | 2.63–2.65 | 7 | U+ | Typically translucent to opaque |
| Rubellite | 1.624–1.644 | 3.02–3.10 | 7.0–7.5 | U− | Deep saturated red-pink stable under incandescent and daylight illumination |
| Ruby | 1.762–1.770 | 3.97–4.05 | 9 | U− | Strong red fluorescence (LW) |
| Sapphire | 1.762–1.770 | 3.95–4.03 | 9 | U− | Strong dichroism |
| Scapolite | 1.540–1.572 | 2.50–2.74 | 6.0–6.5 | U− | Strong orange LW and SW fluorescence — diagnostically useful, shared by few other gems |
| Sinhalite | 1.665–1.712 | 3.47–3.50 | 6.5–7.0 | B− | Diagnostic spectroscope absorption bands at 452, 475, and 493 nm — essentially unique to sinhalite |
| Smoky Quartz | 1.544–1.553 | 2.63–2.65 | 7 | U+ | 'Bull's eye' optic figure |
| Sodalite | 1.483–1.487 | 2.14–2.40 | 5.5–6.0 | AGG | Hackmanite variety shows strong orange LW fluorescence (tenebrescence); plain sodalite is inert to LW UV — lapis lazuli shows only weak orange LW |
| Spessartite Garnet | 1.790–1.815 | 4.12–4.18 | 7.0–7.5 | SR | Bright orange color |
| Sphalerite | 2.369–2.500 | 3.90–4.10 | 3.5–4.0 | SR | Extraordinarily high dispersion |
| Sphene | 1.843–2.110 | 3.45–3.55 | 5.0–5.5 | B+ | Extreme dispersion (fire) |
| Spinel | 1.712–1.736 | 3.57–3.63 | 7.5–8.0 | SR | Singly refractive |
| Star Diopside | 1.664–1.721 | 3.27–3.36 | 5.0–6.0 | B+ | 4-rayed star (not 6-rayed) — caused by two sets of magnetite/ilmenite needles perpendicular in (010) plane |
| Star Garnet | 1.770–1.830 | 3.93–4.30 | 7.0–7.5 | SR | 4-rayed star (occasionally 6-rayed in Idaho material) visible under a direct point light source |
| Star Ruby | 1.762–1.770 | 3.97–4.05 | 9 | U− | 6-rayed star under point light source (asterism) |
| Star Sapphire | 1.762–1.770 | 3.95–4.03 | 9 | U− | 6-rayed star under point light source (asterism) |
| Sunstone | 1.537–1.572 | 2.62–2.65 | 6.0–6.5 | B− | Aventurescence (schiller) — metallic spangling from copper or hematite/goethite platelets |
| Synthetic Color-Change Corundum | 1.762–1.770 | 3.97–4.05 | 9 | U− | RI 1.762–1.778 — immediately distinguishable from true alexandrite (chrysoberyl) at 1.746–1.755 |
| Synthetic Spinel | 1.725–1.728 | 3.61–3.66 | 8 | SR | Strong 'tabby extinction' (checkerboard-like ADR) on polariscope — diagnostic for synthetic origin |
| Taaffeite | 1.719–1.723 | 3.59–3.63 | 8.0–8.5 | U− | Uniaxial negative — THIS separates it from spinel (isotropic, SR) |
| Tanzanite | 1.691–1.700 | 3.35–3.38 | 6.0–7.0 | B+ | Extreme trichroism in heated (commercial) material: a-axis blue-violet, b-axis purple, c-axis green-yellow; unheated material shows brownish-red on one axis |
| Tiger's Eye | 1.544–1.553 | 2.64–2.71 | 7 | AGG | Chatoyancy (cat's eye effect) from parallel fibrous structure |
| Topaz | 1.609–1.643 | 3.49–3.57 | 8 | B+ | High SG for its RI range |
| Tourmaline | 1.624–1.644 | 3.01–3.26 | 7.0–7.5 | U− | Strong pleochroism |
| Tsavorite Garnet | 1.730–1.760 | 3.59–3.65 | 7.0–7.5 | SR | Bright, intense green color |
| Turquoise | 1.610–1.650 | 2.60–2.90 | 5.0–6.0 | AGG | Opaque with waxy luster |
| Watermelon Tourmaline | 1.616–1.650 | 3.00–3.10 | 7.0–7.5 | U− | Distinctive pink/red core with green rim — bicolor zoning visible under loupe and to naked eye |
| White Sapphire | 1.762–1.770 | 3.99–4.01 | 9 | U− | Low dispersion (0.018) — lacks diamond's fire |
| Yellow Diamond | 2.417 | 3.51–3.53 | 10 | SR | Type Ia Cape series: N3 absorption at 415nm produces yellow + strong blue LW fluorescence |
| Yellow Sapphire | 1.762–1.770 | 3.99–4.01 | 9 | U− | RI 1.762 — far higher than citrine (1.544) and golden topaz (1.619) — most reliable separator |
| Zircon | 1.925–1.984 | 4.60–4.80 | 8 | U+ | Extreme facet doubling |
Optic: SR = singly refractive · U = uniaxial · B = biaxial · AGG = aggregate · +/− = optic sign. Values are typical ranges; individual stones vary. Indicators, not certifications.