RI 1.530–1.543 · SG 2.58–2.64 · Aggregate (AGG) · Mohs 6.5–7.0	Agate (var. of Quartz) — Characteristic concentric or parallel banding in multiple colors — primary visual identifier
RI 1.746–1.755 · SG 3.70–3.78 · Biaxial + (DR) · Mohs 9 · strong trichroism	Alexandrite (var. of Chrysoberyl) — Dramatic color change
RI 1.770–1.830 · SG 3.93–4.30 · Singly refractive (SR) · Mohs 7.0–7.5	Almandine Garnet (var. of Garnet) — Deep red color
RI 1.514–1.530 · SG 2.54–2.58 · Biaxial − (DR) · Mohs 6.0–6.5	Amazonite (var. of Feldspar) — Whitish cross-hatch twinning (perthitic grid) visible under magnification — diagnostic
RI 1.539–1.545 · SG 1.05–1.09 · Singly refractive (SR) · Mohs 2.0–2.5 · strong blue LWUV	Amber — Extremely low SG (floats in salt water)
RI 1.544–1.553 · SG 2.63–2.65 · Uniaxial + (DR) · Mohs 7	Amethyst (var. of Quartz) — 'Bull's eye' optic figure
RI 1.544–1.553 · SG 2.63–2.67 · Uniaxial + (DR) · Mohs 7	Ametrine (var. of Quartz) — Distinct bicolor zones — purple amethyst and yellow/orange citrine in same crystal
RI 1.629–1.649 · SG 3.13–3.21 · Biaxial − (DR) · Mohs 7.0–7.5 · strong trichroism	Andalusite — Striking trichroism: green / red-brown / yellow-orange, often visible to naked eye
RI 1.628–1.649 · SG 3.17–3.23 · Uniaxial − (DR) · Mohs 5 · moderate pleochroism	Apatite — Hardness exactly 5.0 (Mohs reference mineral) — scratched by a knife or steel file
RI 1.577–1.583 · SG 2.68–2.78 · Uniaxial − (DR) · Mohs 7.5–8.0	Aquamarine (var. of Beryl) — Low RI and SG
RI 1.544–1.553 · SG 2.64–2.69 · Aggregate (AGG) · Mohs 7	Aventurine (var. of Quartz) — Aventurescence from flat fuchsite (green mica) platelets — visible under magnification
RI 1.757–1.804 · SG 3.64–3.68 · Uniaxial + (DR) · Mohs 6.0–6.5 · moderate pleochroism	Benitoite — Vivid strong blue-white SW UV fluorescence — near-unique diagnostic property
RI 1.440–1.460 · SG 2.09–2.16 · Singly refractive (SR) · Mohs 5.5–6.5	Black Opal (var. of Opal) — Dark body tone (N1–N4 on GIA tone scale) with vivid play-of-color
RI 1.712–1.730 · SG 3.58–3.61 · Singly refractive (SR) · Mohs 8	Black Spinel — Singly refractive
RI 1.530–1.543 · SG 2.58–2.64 · Aggregate (AGG) · Mohs 6.5–7.0	Bloodstone (var. of Quartz) — Dark green opaque chalcedony with distinctive red/orange spots — diagnostically useful visual appearance
RI 2.417 · SG 3.50–3.53 · Singly refractive (SR) · Mohs 10	Blue Diamond (var. of Diamond) — Strong blue phosphorescence after SW UV (5–30+ seconds)
RI 2.417 · SG 3.51–3.53 · Singly refractive (SR) · Mohs 10	Brown Diamond (var. of Diamond) — Most common fancy color — approximately 80% of all diamonds are brown or near-colorless
RI 1.530–1.543 · SG 2.58–2.64 · Aggregate (AGG) · Mohs 6.5–7.0	Carnelian (var. of Quartz) — Translucent orange-red to red-brown body color — primary identifier
RI 2.010–2.100 · SG 6.80–7.10 · Uniaxial + (DR) · Mohs 6.0–7.0 · strong dichroism	Cassiterite — Extremely high specific gravity
RI 1.746–1.755 · SG 3.70–3.78 · Biaxial + (DR) · Mohs 9	Cat's Eye Chrysoberyl (var. of Chrysoberyl) — '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
RI 1.530–1.543 · SG 2.58–2.64 · Aggregate (AGG) · Mohs 6.5–7.0	Chalcedony (var. of Quartz) — Microcrystalline quartz aggregate — RI ~1.535 lower than macrocrystalline quartz (1.544–1.553)
RI 1.550–1.559 · SG 2.54–2.68 · Aggregate (AGG) · Mohs 5.0–6.0	Charoite — Unique fibrous swirling lavender-purple texture with orange (tinaksite) and black (aegirine) inclusions
RI 1.664–1.730 · SG 3.22–3.38 · Biaxial + (DR) · Mohs 5.5–6.5 · moderate pleochroism	Chrome Diopside (var. of Diopside (pyroxene)) — Chelsea filter red — chromium coloring (confirms Cr; does not distinguish from emerald without RI)
RI 1.624–1.660 · SG 3.00–3.20 · Uniaxial − (DR) · Mohs 7.0–7.5 · strong dichroism	Chrome Tourmaline (var. of Tourmaline) — Chelsea filter RED — chromium coloring causes strong red reaction, unlike Fe-colored verdelite
RI 1.625–1.675 · SG 3.13–3.35 · Biaxial + (DR) · Mohs 6	Clinohumite — Biaxial DR (monoclinic) — shows 4 extinction positions per rotation under polariscope; spessartine is isotropic SR
RI 1.746–1.755 · SG 3.70–3.78 · Biaxial + (DR) · Mohs 9 · strong trichroism	Chrysoberyl — High hardness of 8.5
RI 1.460–1.630 · SG 2.00–2.40 · Aggregate (AGG) · Mohs 2.5–7.0	Chrysocolla — Cyan (blue-green) color
RI 1.530–1.543 · SG 2.58–2.64 · Aggregate (AGG) · Mohs 6.5–7.0	Chrysoprase (var. of Quartz) — Apple-green to yellow-green translucent body color caused by nickel — primary identifier
RI 2.905–3.256 · SG 8.00–8.20 · Uniaxial + (DR) · Mohs 2.0–2.5 · strong dichroism	Cinnabar — Extremely high specific gravity
RI 1.544–1.553 · SG 2.63–2.65 · Uniaxial + (DR) · Mohs 7	Citrine (var. of Quartz) — 'Bull's eye' optic figure
RI 1.740–1.765 · SG 3.65–3.80 · Singly refractive (SR) · Mohs 7.0–7.5	Color-Change Garnet (var. of Garnet) — Singly refractive (isotropic) — stays dark under polariscope; alexandrite is doubly refractive and blinks
RI 1.762–1.770 · SG 3.99–4.01 · Uniaxial − (DR) · Mohs 9 · strong dichroism	Color-Change Sapphire (var. of Corundum) — RI 1.762 — higher than alexandrite (1.746) — primary physical separator
RI 1.486–1.658 · SG 2.60–2.70 · Aggregate (AGG) · Mohs 3.0–4.0	Coral — Organic origin (skeletal material)
RI 2.150–2.180 · SG 5.60–6.00 · Singly refractive (SR) · Mohs 8.0–8.5	Cubic Zirconia — Very high SG (5.6–6.0) — heaviest common gem material; dramatically heavier than any natural simulant
RI 2.849 · SG 5.85–6.15 · Singly refractive (SR) · Mohs 3.5–4.0	Cuprite — Deep red color with metallic/adamantine luster
RI 1.627–1.636 · SG 2.97–3.02 · Biaxial − (DR) · Mohs 7.0–7.5 · strong blue LWUV	Danburite — Strong sky-blue LW and SW fluorescence — highly distinctive
RI 1.880–1.895 · SG 3.82–3.88 · Singly refractive (SR) · Mohs 6.5–7.0 · red under Chelsea filter	Demantoid Garnet (var. of Garnet) — Dispersion higher than diamond
RI 2.417 · SG 3.50–3.53 · Singly refractive (SR) · Mohs 10	Diamond — Adamantine luster
RI 1.685–1.752 · SG 3.30–3.39 · Biaxial + (DR) · Mohs 6.5–7.0 · strong trichroism	Diaspore — Color change: green (daylight) → champagne/pink (incandescent) — distinct from alexandrite green-to-red
RI 1.565–1.602 · SG 2.67–2.78 · Uniaxial − (DR) · Mohs 7.5–8.0 · strong dichroism	Emerald (var. of Beryl) — Shows red through Chelsea filter
RI 1.663–1.673 · SG 3.20–3.30 · Biaxial + (DR) · Mohs 5.0–5.5 · strong trichroism	Enstatite (var. of Enstatite (pyroxene — MgSiO₃)) — Chelsea filter red reaction — chromium-bearing; shared with chrome diopside but different RI/SG
RI 1.370–1.470 · SG 1.98–2.20 · Singly refractive (SR) · Mohs 5.5–6.5	Fire Opal (var. of Opal) — Orange-red body color with no play-of-color (typically)
RI 1.432–1.436 · SG 3.17–3.19 · Singly refractive (SR) · Mohs 4 · strong blue LWUV	Fluorite — Low hardness (4)
RI 1.577–1.583 · SG 2.68–2.80 · Uniaxial − (DR) · Mohs 7.5–8.0	Goshenite (var. of Beryl) — RI 1.577–1.583 clearly lower than white sapphire (1.762) and white topaz (1.619–1.627)
RI 1.590–1.623 · SG 2.85–2.98 · Biaxial − (DR) · Mohs 7.0–7.5 · strong trichroism	Grandidierite — Extreme trichroism: dark blue-green / pale blue / colorless-to-pale-yellow — essentially diagnostic for this gem
RI 1.616–1.650 · SG 3.00–3.10 · Uniaxial − (DR) · Mohs 7.0–7.5 · strong dichroism	Green Tourmaline (var. of Tourmaline) — Much lower RI (1.624-1.644) than green sapphire (1.762) or tsavorite (1.740)
RI 1.734–1.759 · SG 3.57–3.73 · Singly refractive (SR) · Mohs 6.5–7.5	Grossular Garnet (var. of Garnet (grossular — calcium aluminum silicate)) — Colorless (leuco) to delicate mint green — paler, softer green than tsavorite (which is rich saturated green)
RI 1.483–1.487 · SG 2.14–2.40 · Singly refractive (SR) · Mohs 5.5–6.0 · strong orange-pink LWUV	Hackmanite (var. of Sodalite) — Tenebrescence — reversible color change under UV: colorless/pale in daylight, purple-pink after SW UV exposure
RI 1.494–1.509 · SG 2.44–2.50 · Singly refractive (SR) · Mohs 5.5–6.0 · strong orange-pink LWUV	Hauyne — Extremely low RI 1.494–1.509 — below refractometer scale upper limit; far below sapphire (1.762), tourmaline (1.614–1.666), and topaz (1.609–1.643)
RI 1.544–1.553 · SG 2.64–2.71 · Aggregate (AGG) · Mohs 7	Hawk's Eye (var. of Quartz) — Blue-grey to blue-green color from unoxidized crocidolite (unlike golden Tiger's Eye)
RI 1.568–1.580 · SG 2.68–2.74 · Uniaxial − (DR) · Mohs 7.5–8.0	Heliodor (var. of Beryl) — Beryl RI and SG — separates from all yellow simulants
RI 2.940–3.220 · SG 4.95–5.30 · Uniaxial − (DR) · Mohs 5.5–6.5	Hematite — High metallic luster
RI 1.730–1.760 · SG 3.59–3.65 · Singly refractive (SR) · Mohs 7.0–7.5	Hessonite Garnet (var. of Garnet) — Treacly or roiled internal appearance
RI 1.660–1.676 · SG 3.15–3.21 · Biaxial + (DR) · Mohs 6.5–7.0 · strong trichroism	Hiddenite (var. of Spodumene) — Perfect cleavage in two directions — chips easily along cleavage planes
RI 1.583–1.608 · SG 2.45–2.58 · Aggregate (AGG) · Mohs 4 · strong blue LWUV	Howlite — Natural white/gray color with distinctive gray-black veining resembling spider-web turquoise matrix
RI 1.624–1.644 · SG 3.02–3.10 · Uniaxial − (DR) · Mohs 7.0–7.5 · strong dichroism	Indicolite (var. of Tourmaline) — Strong dichroism — dark blue vs. light blue/colorless when rotated
RI 1.535–1.547 · SG 2.57–2.66 · Biaxial − (DR) · Mohs 7.0–7.5 · strong trichroism	Iolite (var. of Cordierite) — Extreme pleochroism
RI 1.535–1.540 · SG 1.70–1.98 · Aggregate (AGG) · Mohs 2.5–3.0	Ivory (var. of Organic) — Schreger lines — cross-hatched pattern visible on cross-section: elephant <115°, mammoth >115°
RI 1.654–1.688 · SG 3.24–3.43 · Aggregate (AGG) · Mohs 6.5–7.0	Jadeite — Interlocking blocky granular texture (orange-peel surface on polished stones)
RI 1.640–1.680 · SG 1.10–1.40 · Singly refractive (SR) · Mohs 2.5–4.0	Jet — Extremely low SG 1.10–1.40 — nearly floats in saturated salt water; noticeably light in hand
RI 1.660–1.699 · SG 3.27–3.45 · Biaxial − (DR) · Mohs 7 · strong trichroism	Kornerupine — Strong trichroism (colorless / brown-yellow / green) — visible to naked eye in some specimens
RI 1.660–1.676 · SG 3.15–3.21 · Biaxial + (DR) · Mohs 6.5–7.0 · strong trichroism	Kunzite (var. of Spodumene) — Strong pleochroism
RI 1.712–1.734 · SG 3.53–3.68 · Biaxial − (DR) · Mohs 4.5–7.0 · strong trichroism	Kyanite — Variable hardness by direction — 4.5 along crystal length, 6.5–7.0 across (unique among gemstones)
RI 1.559–1.573 · SG 2.68–2.75 · Biaxial − (DR) · Mohs 6.0–6.5	Labradorite (var. of Feldspar) — Labradorescence
RI 1.500–1.670 · SG 2.70–2.90 · Aggregate (AGG) · Mohs 5.0–6.0	Lapis Lazuli — Intense blue color
RI 1.595–1.640 · SG 2.74–2.90 · Biaxial + (DR) · Mohs 4.5–5.0	Larimar (var. of Pectolite) — Sky blue color with distinctive white radiating or cloud-like patterns unique to pectolite
RI 1.780–1.815 · SG 3.90–4.10 · Singly refractive (SR) · Mohs 6.0–7.5	Low-Type Zircon — Metamict state (damaged crystal lattice)
RI 1.655–1.909 · SG 3.60–4.05 · Biaxial − (DR) · Mohs 3.5–4.0	Malachite — Concentric banded green patterns — swirling bull's-eye and wavy bands are the primary visual identifier
RI 1.730–1.780 · SG 3.65–3.87 · Singly refractive (SR) · Mohs 7.0–7.5	Malaya / Umbalite Garnet (var. of Garnet (pyrope-spessartite solid solution)) — Peachy orange-pink color unusual in the gem world — distinct from orange sapphire and padparadscha
RI 2.648–2.691 · SG 3.20–3.22 · Uniaxial + (DR) · Mohs 9	Moissanite — All commercial moissanite is synthetic — no natural gem exists
RI 1.480–1.510 · SG 2.32–2.38 · Singly refractive (SR) · Mohs 6	Moldavite (var. of Silica Glass (tektite)) — Unique sculptured/etched exterior surface with flow ridges, pitting, and corrosion features
RI 1.518–1.526 · SG 2.56–2.59 · Biaxial − (DR) · Mohs 6.0–6.5	Moonstone (var. of Feldspar) — Adularescence (schiller)
RI 1.572–1.600 · SG 2.71–2.85 · Uniaxial − (DR) · Mohs 7.5–8.0 · moderate pleochroism	Morganite (var. of Beryl) — Higher RI and SG than other beryls
RI 1.600–1.627 · SG 2.90–3.03 · Aggregate (AGG) · Mohs 6.0–6.5	Nephrite — Fibrous, felted texture
RI 1.480–1.510 · SG 2.30–2.60 · Singly refractive (SR) · Mohs 5.0–5.5	Obsidian — Amorphous volcanic glass
RI 1.530–1.543 · SG 2.58–2.64 · Aggregate (AGG) · Mohs 6.5–7.0	Onyx (var. of Quartz) — Opaque black body color — most common form in trade
RI 1.370–1.470 · SG 1.98–2.25 · Singly refractive (SR) · Mohs 5.5–6.5	Opal — Play-of-color
RI 1.762–1.770 · SG 3.95–4.03 · Uniaxial − (DR) · Mohs 9 · strong dichroism	Padparadscha Sapphire (var. of Corundum) — Unique pinkish-orange to orangey-pink hue — named 'lotus flower' color
RI 1.624–1.644 · SG 3.02–3.10 · Uniaxial − (DR) · Mohs 7.0–7.5 · strong dichroism	Paraiba Tourmaline (var. of Tourmaline) — Electric neon blue-green — unmistakable when genuine
RI 1.520–1.690 · SG 2.60–2.85 · Aggregate (AGG) · Mohs 2.5–4.5	Pearl — Characteristic orient/luster
RI 1.654–1.690 · SG 3.27–3.48 · Biaxial + (DR) · Mohs 6.5–7.0	Peridot (var. of Olivine) — Very strong facet doubling
RI 1.654–1.670 · SG 2.95–2.97 · Uniaxial + (DR) · Mohs 7.5–8.0	Phenakite — Higher RI than quartz (1.544) and higher than white topaz (1.619–1.627) — phenakite RI 1.654–1.670 exceeds topaz
RI 1.534–1.540 · SG 2.58–2.64 · Aggregate (AGG) · Mohs 6.5–7.0	Pietersite — Chaotic angular brecciated pattern — blue, gold, and red-brown zones in broken fragments cemented by later silica
RI 2.417 · SG 3.51–3.53 · Singly refractive (SR) · Mohs 10	Pink Diamond (var. of Diamond) — Single refractive index 2.417 — highest of any natural gemstone
RI 1.762–1.770 · SG 3.98–4.01 · Uniaxial − (DR) · Mohs 9 · strong dichroism	Pink Sapphire (var. of Corundum) — Strong red LW fluorescence (especially Myanmar material) — stronger than rubellite
RI 1.544–1.553 · SG 2.63–2.67 · Uniaxial + (DR) · Mohs 7	Prasiolite (var. of Quartz) — Pale leek-green to mint green color distinct from deeper greens of tourmaline or peridot
RI 1.611–1.665 · SG 2.82–2.94 · Biaxial + (DR) · Mohs 6.0–6.5	Prehnite — Pale apple-green to yellow-green color typical
RI 1.730–1.760 · SG 3.62–3.87 · Singly refractive (SR) · Mohs 7.0–7.5	Pyrope Garnet (var. of Garnet) — Blood-red color
RI 1.559–1.572 · SG 2.68–2.72 · Biaxial − (DR) · Mohs 6.0–6.5	Rainbow Moonstone (var. of Feldspar) — RI 1.559–1.572 — ABOVE true moonstone (1.518–1.526); this single test identifies it
RI 1.564–1.574 · SG 2.66–2.70 · Uniaxial − (DR) · Mohs 7.5–8.0 · strong dichroism	Red Beryl (var. of Beryl) — Extremely rare — essentially restricted to Wah Wah Mountains, Utah; faceted stones over 1ct are exceptional
RI 1.600–1.820 · SG 3.40–3.70 · Uniaxial − (DR) · Mohs 3.5–4.0	Rhodochrosite — Extreme birefringence 0.220 — back facets visibly doubled even to the naked eye
RI 1.740–1.770 · SG 3.74–3.94 · Singly refractive (SR) · Mohs 7.0–7.5	Rhodolite Garnet (var. of Garnet) — Distinctive purplish-red to raspberry red color
RI 1.716–1.752 · SG 3.57–3.76 · Biaxial + (DR) · Mohs 5.5–6.5	Rhodonite — Pink to red color
RI 1.544–1.553 · SG 2.63–2.65 · Uniaxial + (DR) · Mohs 7	Rock Crystal (var. of Quartz) — 'Bull's eye' optic figure
RI 1.544–1.553 · SG 2.63–2.65 · Uniaxial + (DR) · Mohs 7	Rose Quartz (var. of Quartz) — Typically translucent to opaque
RI 1.624–1.644 · SG 3.02–3.10 · Uniaxial − (DR) · Mohs 7.0–7.5 · strong dichroism	Rubellite (var. of Tourmaline) — Deep saturated red-pink stable under incandescent and daylight illumination
RI 1.762–1.770 · SG 3.97–4.05 · Uniaxial − (DR) · Mohs 9 · strong dichroism	Ruby (var. of Corundum) — Strong red fluorescence (LW)
RI 1.762–1.770 · SG 3.95–4.03 · Uniaxial − (DR) · Mohs 9 · strong dichroism	Sapphire (var. of Corundum) — Strong dichroism
RI 1.540–1.572 · SG 2.50–2.74 · Uniaxial − (DR) · Mohs 6.0–6.5 · strong dichroism	Scapolite — Strong orange LW and SW fluorescence — diagnostically useful, shared by few other gems
RI 1.560–1.571 · SG 2.44–2.62 · Aggregate (AGG) · Mohs 3.0–4.0	Serpentine — RI 1.560–1.571 — dramatically lower than jadeite (1.654–1.688) and nephrite (1.600–1.627)
RI 1.665–1.712 · SG 3.47–3.50 · Biaxial − (DR) · Mohs 6.5–7.0 · strong trichroism	Sinhalite — Diagnostic spectroscope absorption bands at 452, 475, and 493 nm — essentially unique to sinhalite
RI 1.544–1.553 · SG 2.63–2.65 · Uniaxial + (DR) · Mohs 7	Smoky Quartz (var. of Quartz) — 'Bull's eye' optic figure
RI 1.483–1.487 · SG 2.14–2.40 · Aggregate (AGG) · Mohs 5.5–6.0	Sodalite — Hackmanite variety shows strong orange LW fluorescence (tenebrescence); plain sodalite is inert to LW UV — lapis lazuli shows only weak orange LW
RI 1.790–1.815 · SG 4.12–4.18 · Singly refractive (SR) · Mohs 7.0–7.5	Spessartite Garnet (var. of Garnet) — Bright orange color
RI 2.369–2.500 · SG 3.90–4.10 · Singly refractive (SR) · Mohs 3.5–4.0	Sphalerite — Extraordinarily high dispersion
RI 1.843–2.110 · SG 3.45–3.55 · Biaxial + (DR) · Mohs 5.0–5.5 · strong trichroism	Sphene (var. of Titanite) — Extreme dispersion (fire)
RI 1.712–1.736 · SG 3.57–3.63 · Singly refractive (SR) · Mohs 7.5–8.0	Spinel — Singly refractive
RI 1.664–1.721 · SG 3.27–3.36 · Biaxial + (DR) · Mohs 5.0–6.0	Star Diopside (var. of Diopside) — 4-rayed star (not 6-rayed) — caused by two sets of magnetite/ilmenite needles perpendicular in (010) plane
RI 1.770–1.830 · SG 3.93–4.30 · Singly refractive (SR) · Mohs 7.0–7.5	Star Garnet (var. of Garnet) — 4-rayed star (occasionally 6-rayed in Idaho material) visible under a direct point light source
RI 1.762–1.770 · SG 3.97–4.05 · Uniaxial − (DR) · Mohs 9 · strong dichroism	Star Ruby (var. of Corundum) — 6-rayed star under point light source (asterism)
RI 1.762–1.770 · SG 3.95–4.03 · Uniaxial − (DR) · Mohs 9 · strong dichroism	Star Sapphire (var. of Corundum) — 6-rayed star under point light source (asterism)
RI 1.607–1.610 · SG 2.74–2.80 · Uniaxial − (DR) · Mohs 5.5–6.5	Sugilite — Vivid purple to violet color, often with rich grape-juice saturation
RI 1.537–1.572 · SG 2.62–2.65 · Biaxial − (DR) · Mohs 6.0–6.5	Sunstone (var. of Feldspar) — Aventurescence (schiller) — metallic spangling from copper or hematite/goethite platelets
RI 1.762–1.770 · SG 3.97–4.05 · Uniaxial − (DR) · Mohs 9 · strong dichroism	Synthetic Color-Change Corundum (var. of Corundum (synthetic)) — RI 1.762–1.778 — immediately distinguishable from true alexandrite (chrysoberyl) at 1.746–1.755
RI 1.725–1.728 · SG 3.61–3.66 · Singly refractive (SR) · Mohs 8 · strong blue-white LWUV	Synthetic Spinel (var. of Spinel (synthetic)) — Strong 'tabby extinction' (checkerboard-like ADR) on polariscope — diagnostic for synthetic origin
RI 1.719–1.723 · SG 3.59–3.63 · Uniaxial − (DR) · Mohs 8.0–8.5	Taaffeite — Uniaxial negative — THIS separates it from spinel (isotropic, SR)
RI 1.691–1.700 · SG 3.35–3.38 · Biaxial + (DR) · Mohs 6.0–7.0 · strong trichroism	Tanzanite (var. of Zoisite) — 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
RI 1.691–1.704 · SG 3.10–3.38 · Biaxial + (DR) · Mohs 6.0–7.0	Thulite (var. of Zoisite) — Opaque to translucent pink zoisite — massive habit, typically cabochon or ornamental
RI 1.544–1.553 · SG 2.64–2.71 · Aggregate (AGG) · Mohs 7	Tiger's Eye (var. of Quartz) — Chatoyancy (cat's eye effect) from parallel fibrous structure
RI 1.609–1.643 · SG 3.49–3.57 · Biaxial + (DR) · Mohs 8 · moderate pleochroism	Topaz — High SG for its RI range
RI 1.624–1.644 · SG 3.01–3.26 · Uniaxial − (DR) · Mohs 7.0–7.5 · strong dichroism	Tourmaline — Strong pleochroism
RI 1.730–1.760 · SG 3.59–3.65 · Singly refractive (SR) · Mohs 7.0–7.5 · red under Chelsea filter	Tsavorite Garnet (var. of Garnet) — Bright, intense green color
RI 1.610–1.650 · SG 2.60–2.90 · Aggregate (AGG) · Mohs 5.0–6.0	Turquoise — Opaque with waxy luster
RI 1.560–1.590 · SG 2.40–2.60 · Biaxial − (DR) · Mohs 3.5–4.5	Variscite — Apple green to pale green color — lacks the blue tint characteristic of turquoise
RI 1.616–1.650 · SG 3.00–3.10 · Uniaxial − (DR) · Mohs 7.0–7.5 · strong dichroism	Watermelon Tourmaline (var. of Tourmaline) — Distinctive pink/red core with green rim — bicolor zoning visible under loupe and to naked eye
RI 1.762–1.770 · SG 3.99–4.01 · Uniaxial − (DR) · Mohs 9	White Sapphire (var. of Corundum) — Low dispersion (0.018) — lacks diamond's fire
RI 1.833 · SG 4.50–4.60 · Singly refractive (SR) · Mohs 8.0–8.5	YAG (var. of Synthetic Garnet) — RI 1.833 — fixed, single value (isotropic); immediately diagnostic
RI 2.417 · SG 3.51–3.53 · Singly refractive (SR) · Mohs 10	Yellow Diamond (var. of Diamond) — Type Ia Cape series: N3 absorption at 415nm produces yellow + strong blue LW fluorescence
RI 1.762–1.770 · SG 3.99–4.01 · Uniaxial − (DR) · Mohs 9	Yellow Sapphire (var. of Corundum) — RI 1.762 — far higher than citrine (1.544) and golden topaz (1.619) — most reliable separator
RI 1.925–1.984 · SG 4.60–4.80 · Uniaxial + (DR) · Mohs 8 · moderate pleochroism	Zircon — Extreme facet doubling