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Gemstone properties chart

50 commonly encountered gems: RI, SG, hardness, birefringence, crystal system, cleavage

A single-page reference table of the six diagnostic gemmological constants (refractive index, specific gravity, Mohs hardness, birefringence, crystal system, and cleavage) for 50 of the most commonly encountered gemstones in commercial and FGA-curriculum practice. Values are stated as Webster and Read report them, with ranges where compositional variation is significant (e.g. tourmaline 1.624–1.644 reflecting elbaite chemistry). Cabochon and aggregate readings are flagged separately because the refractometer reports a spot reading rather than a birefringence in those cases. For interactive lookup against your own measured values, try the RI calculator or the Gem Identifier, both of which run entirely in your browser. For deeper coverage of each species and full mineralogical context, follow the species links on the Mineral Database (159 expressions across 121 families). See the source-citation note below the table for our data lineage.

Reference table: 50 gemstones

Name Species Formula Crystal system RI Birefringence SG Hardness Cleavage
Agate Quartz (chalcedony) SiO₂ Trigonal 1.535–1.539 0.004 (aggregate) 2.58–2.64 6.5–7 None
Alexandrite Chrysoberyl BeAl₂O₄ Orthorhombic 1.745–1.755 0.008–0.010 3.70–3.73 8.5 Distinct (1 direction)
Almandine garnet Garnet Fe₃Al₂(SiO₄)₃ Cubic 1.770–1.830 None 3.95–4.30 7–7.5 None
Amazonite Feldspar (microcline) KAlSi₃O₈ Triclinic 1.522–1.530 0.008 2.56–2.58 6–6.5 Perfect (2 directions)
Amber Fossil resin C₁₀H₁₆O Amorphous 1.539–1.545 None 1.05–1.10 2–2.5 None
Amethyst Quartz SiO₂ Trigonal 1.544–1.553 0.009 2.65 7 None
Andalusite Andalusite Al₂SiO₅ Orthorhombic 1.634–1.643 0.007–0.013 3.12–3.18 7–7.5 Distinct (1 direction)
Apatite Apatite Ca₅(PO₄)₃(F,Cl,OH) Hexagonal 1.628–1.649 0.002–0.013 3.16–3.23 5 Indistinct
Aquamarine Beryl Be₃Al₂Si₆O₁₈ Hexagonal 1.564–1.596 0.005–0.009 2.68–2.91 7.5–8 Indistinct
Aventurine Quartz SiO₂ Trigonal 1.544–1.553 0.009 (aggregate) 2.64–2.69 7 None
Beryl (heliodor) Beryl Be₃Al₂Si₆O₁₈ Hexagonal 1.566–1.591 0.005–0.008 2.66–2.83 7.5–8 Indistinct
Carnelian Quartz (chalcedony) SiO₂ Trigonal 1.535–1.539 0.004 (aggregate) 2.58–2.64 6.5–7 None
Chrysoberyl Chrysoberyl BeAl₂O₄ Orthorhombic 1.746–1.755 0.008–0.011 3.71–3.75 8.5 Distinct (1 direction)
Chrysoprase Quartz (chalcedony) SiO₂ Trigonal 1.535–1.539 0.004 (aggregate) 2.58–2.64 6.5–7 None
Citrine Quartz SiO₂ Trigonal 1.544–1.553 0.009 2.65 7 None
Coral (precious) Calcium carbonate CaCO₃ Trigonal 1.486–1.658 0.160 (aggregate) 2.60–2.70 3–4 None
Demantoid garnet Garnet (andradite) Ca₃Fe₂(SiO₄)₃ Cubic 1.880–1.889 None 3.82–3.85 6.5–7 None
Diamond Diamond C Cubic 2.417 None 3.51–3.53 10 Perfect (4 directions)
Diopside Pyroxene CaMgSi₂O₆ Monoclinic 1.664–1.730 0.024–0.030 3.22–3.40 5–6 Good (2 directions)
Emerald Beryl Be₃Al₂Si₆O₁₈ Hexagonal 1.570–1.590 0.005–0.009 2.67–2.78 7.5–8 Indistinct
Fluorite Fluorite CaF₂ Cubic 1.433–1.435 None 3.18 4 Perfect (4 directions)
Hessonite garnet Garnet (grossular) Ca₃Al₂(SiO₄)₃ Cubic 1.736–1.745 None 3.60–3.68 7–7.5 None
Iolite Cordierite (Mg,Fe)₂Al₃[AlSi₅O₁₈] Orthorhombic 1.522–1.578 0.008–0.012 2.57–2.66 7–7.5 Distinct (1 direction)
Jadeite Pyroxene NaAlSi₂O₆ Monoclinic 1.652–1.688 0.012 (aggregate) 3.30–3.38 6.5–7 Good (none in aggregate)
Kunzite Spodumene LiAlSi₂O₆ Monoclinic 1.660–1.681 0.014–0.016 3.15–3.21 6.5–7 Perfect (2 directions)
Labradorite Feldspar (Ca,Na)(Al,Si)₄O₈ Triclinic 1.559–1.572 0.008–0.010 2.69–2.72 6–6.5 Perfect (2 directions)
Lapis lazuli Rock (lazurite + others) (Na,Ca)₈(Al,Si)₁₂O₂₄(S,SO₄) Cubic (aggregate) 1.50 (mean) None (aggregate) 2.50–3.00 5–6 None
Malachite Malachite Cu₂CO₃(OH)₂ Monoclinic 1.655–1.909 0.254 (aggregate) 3.95–4.10 3.5–4 Perfect (aggregate)
Moonstone Feldspar (orthoclase) KAlSi₃O₈ Monoclinic 1.518–1.526 0.005–0.008 2.56–2.59 6–6.5 Perfect (2 directions)
Morganite Beryl Be₃Al₂Si₆O₁₈ Hexagonal 1.585–1.594 0.008–0.009 2.71–2.90 7.5–8 Indistinct
Opal (precious) Hydrated silica SiO₂·nH₂O Amorphous 1.37–1.47 None 1.98–2.25 5.5–6.5 None
Pearl Aragonite (organic) CaCO₃ Orthorhombic (aggregate) 1.530–1.685 0.155 (aggregate) 2.66–2.78 2.5–4 None
Peridot Olivine (forsterite) (Mg,Fe)₂SiO₄ Orthorhombic 1.654–1.690 0.033–0.038 3.27–3.48 6.5–7 Indistinct
Pyrope garnet Garnet Mg₃Al₂(SiO₄)₃ Cubic 1.714–1.760 None 3.65–3.87 7–7.5 None
Quartz (rock crystal) Quartz SiO₂ Trigonal 1.544–1.553 0.009 2.65 7 None
Rhodochrosite Rhodochrosite MnCO₃ Trigonal 1.578–1.840 0.201–0.220 3.45–3.70 3.5–4 Perfect (3 directions)
Rhodolite garnet Garnet (pyrope-almandine) (Mg,Fe)₃Al₂(SiO₄)₃ Cubic 1.745–1.760 None 3.74–3.94 7–7.5 None
Rhodonite Rhodonite MnSiO₃ Triclinic 1.711–1.751 0.011–0.014 3.40–3.74 5.5–6.5 Perfect (2 directions)
Ruby Corundum Al₂O₃ Trigonal 1.762–1.770 0.008–0.009 3.99–4.05 9 None (parting)
Sapphire (blue) Corundum Al₂O₃ Trigonal 1.762–1.770 0.008–0.009 3.95–4.03 9 None (parting)
Sphene (titanite) Titanite CaTiSiO₅ Monoclinic 1.900–2.034 0.105–0.135 3.45–3.55 5–5.5 Distinct (1 direction)
Spinel Spinel MgAl₂O₄ Cubic 1.712–1.736 None 3.58–3.61 8 None
Spodumene (hiddenite) Spodumene LiAlSi₂O₆ Monoclinic 1.660–1.681 0.014–0.016 3.16–3.20 6.5–7 Perfect (2 directions)
Sunstone Feldspar (oligoclase) (Na,Ca)(Al,Si)₄O₈ Triclinic 1.537–1.547 0.007–0.010 2.64–2.66 6–6.5 Perfect (2 directions)
Tanzanite Zoisite Ca₂Al₃(SiO₄)₃(OH) Orthorhombic 1.691–1.700 0.009 3.10–3.38 6–7 Perfect (1 direction)
Topaz Topaz Al₂SiO₄(F,OH)₂ Orthorhombic 1.609–1.638 0.008–0.011 3.49–3.57 8 Perfect (1 direction)
Tourmaline (elbaite) Tourmaline Na(Li,Al)₃Al₆(BO₃)₃Si₆O₁₈(OH)₄ Trigonal 1.624–1.644 0.014–0.024 3.00–3.10 7–7.5 None
Tsavorite garnet Garnet (grossular) Ca₃Al₂(SiO₄)₃ Cubic 1.734–1.760 None 3.60–3.68 7–7.5 None
Turquoise Turquoise CuAl₆(PO₄)₄(OH)₈·4H₂O Triclinic 1.610–1.650 (mean 1.62) 0.040 (aggregate) 2.60–2.85 5–6 None
Zircon (high) Zircon ZrSiO₄ Tetragonal 1.925–1.984 0.058–0.059 4.60–4.80 7.5 Indistinct

Showing 50 gemstones. Hardness is on the Mohs scale; SG is dimensionless (water = 1.000 at 4 °C); RI is reported under sodium light (589 nm).

How to use this table

  1. Identify by RI first. Measure the unknown stone on a refractometer (sodium light, contact fluid n=1.81). The midpoint reading locates a row band in this table to roughly ±0.005.
  2. Distinguish by birefringence. Rotate the stone through 180°. A singly-refractive stone matches an isotropic row ("None"); a doubly-refractive stone gives a birefringence value that further narrows the candidates.
  3. Confirm with SG. Hydrostatic weighing or heavy liquids resolves ambiguities between species with overlapping RI (e.g. emerald 2.67–2.78 vs aquamarine 2.68–2.91, which are closely overlapping; hessonite 3.60–3.68 vs tsavorite 3.60–3.68 needs colour and inclusion analysis).
  4. Check cleavage for cutting / care. Topaz (perfect 1-direction), diamond (perfect 4-direction), and spodumene (perfect 2-direction) require careful setting and avoid ultrasonic cleaning.

Related learn material

Each constant in this table has a dedicated tutorial in the Learn section explaining how it is measured and what its diagnostic limits are:

Sources and methodology

Property values are taken from these standard FGA-curriculum references and cross-checked against the gemmology.dev mineral database:

  • Reade, P. (ed.) (2018). Webster's Gems: Their Sources, Descriptions and Identification, 7th ed., Routledge.
  • Read, P. G. (2014). Gemmology, 3rd ed., Routledge.
  • O'Donoghue, M. (ed.) (2006). Gems: Their Sources, Descriptions and Identification, 6th ed., Butterworth-Heinemann.
  • Schumann, W. (2013). Gemstones of the World, 5th ed., Sterling Publishing.
  • Laboratory Manual Harmonisation Committee (2010). LMHC Standards and Terminology.

Source data on this site is published under CC BY-SA 4.0; the supporting tooling and mineral database are MIT-licensed.