Refractive Index of Gemstones — Complete Reference Chart

Last updated: April 2026

RI values for 132 gemstone species, sorted high to low by maximum RI. Includes birefringence and optic character.

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Common RI lookups: Diamond 2.417  ·  Moissanite 2.648–2.691  ·  Cubic Zirconia 2.150–2.180  ·  Ruby 1.762–1.770  ·  Sapphire 1.762–1.770  ·  Emerald 1.565–1.602  ·  Morganite 1.572–1.600  ·  Topaz 1.609–1.643  ·  Rhodolite Garnet 1.740–1.770  ·  Tourmaline 1.624–1.644

Refractive index (RI) measures how much a gemstone bends light passing through it. It is one of the most reliable diagnostic constants in gemology and is measured with a refractometer — a standard piece of field and laboratory equipment. The RI of a mineral is a physical constant determined by its atomic structure and chemical composition, making it far more reliable than appearance or color for identification purposes.

Gemologists use a spot reading or a moving shadow technique on a refractometer. A singly refractive (SR) stone produces one reading, while doubly refractive (DR) stones produce two shadow edges — giving both the minimum and maximum RI values and, by subtraction, the birefringence. Birefringence causes facet doubling visible under a loupe (notably strong in zircon and calcite), which is itself a diagnostic indicator.

When identifying an unknown stone, always record both readings on a DR stone. Combined with specific gravity, the RI reading typically narrows candidates to one or two species. The table below runs from fire opal at 1.370–1.470 up to cinnabar at 2.905–3.256 — but a standard refractometer stops at about 1.81, so the 22 species whose range reaches past that point (diamond 2.417, moissanite 2.648–2.691 and cubic zirconia 2.150–2.180 among them) give you an over-the-limit reading instead of a number.

How far the instrument reaches, and what it cannot tell you Every value in the table below, on the scale a refractometer actually prints. The curve counts how many species are still possible after one reading — and the second, higher curve is the price of a cabochon spot reading, which is twice as coarse. 0 5 10 15 20 25 SPECIES STILL POSSIBLE solid: faceted reading, ±0.010 · dashed: spot reading, ±0.020 21 species 1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.75 1.80 REFRACTIVE INDEX · THE PRINTED SCALE Opal Quartz Beryl Tourmaline Peridot Spinel Corundum 1.81 · the contact liquid The scale stops here. A stone above this reads "over the limit" — the field goes dark to the bottom and the instrument reports no number at all, only that the index is higher than the liquid. 16 of the 132 rows in this table are entirely beyond it, including: Diamond Moissanite Cubic Zirconia Sphene Cassiterite Cinnabar A value you can look up here is not the same as a value you can read. The table lists both sides of the wall and looks identical either way. A further 6 rows straddle it — one edge on the scale, one edge gone — so a doubly refractive stone can show a single crisp line and be mistaken for a singly refractive one. Almandine garnet, malachite and rhodochrosite are the named cases. All 132 rows of gems.json, 69 distinct species; 116 of them (88%) have some part of their range on the printed scale. Tolerances are the identification engine's own — ±0.010 for a faceted shadow-edge reading, ±0.020 for a cabochon spot reading (decision_engine.dart). The 1.81 ceiling is the CONTACT LIQUID's index, not the prism's, and bottles of 1.79 also exist — the label is the authority, and on a 1.79 bottle the wall moves left. Opal's range begins at 1.37, below the printed floor, so its band runs off the left of the axis. Scale floor 1.40 is the published working range of a standard gem refractometer; Gem-A's own instrument prints down to 1.30.

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Gem RI Min RI Max Birefringence Optic Character SG (typical)
Cinnabar 2.905 3.256 0.351 Uniaxial (+) 8.10
Hematite 2.940 3.220 0.280 Uniaxial (−) 5.28
Cuprite 2.849 2.849 — SR 6.14
Moissanite 2.648 2.691 0.043 Uniaxial (+) 3.22
Blue Diamond 2.417 2.417 — SR 3.52
Brown Diamond 2.417 2.417 — SR 3.52
Diamond 2.417 2.417 — SR 3.52
Pink Diamond 2.417 2.417 — SR 3.52
Yellow Diamond 2.417 2.417 — SR 3.52
Sphalerite 2.369 2.371 — SR 4.09
Cubic Zirconia 2.150 2.180 — SR 5.80
Sphene 1.843 2.110 0.100 Biaxial (+) 3.53
Cassiterite 1.997 2.093 0.096 Uniaxial (+) 6.95
Zircon 1.925 1.984 0.059 Uniaxial (+) 4.69
Malachite 1.655 1.909 0.254 Biaxial (−) 3.80
Demantoid Garnet 1.880 1.895 — SR 3.84
YAG 1.833 1.833 — SR 4.55
Almandine Garnet 1.770 1.830 — SR 4.05
Star Garnet 1.770 1.830 — SR 4.05
Rhodochrosite 1.600 1.820 0.220 Uniaxial (−) 3.50
Low-Type Zircon 1.780 1.815 — SR 4.00
Spessartite Garnet 1.790 1.815 — SR 4.15
Benitoite 1.757 1.804 0.047 Uniaxial (+) 3.66
Malaya / Umbalite Garnet 1.730 1.780 — SR 3.75
Color-Change Sapphire 1.762 1.770 0.008 Uniaxial (−) 4.00
Padparadscha Sapphire 1.762 1.770 0.008 Uniaxial (−) 4.00
Pink Sapphire 1.762 1.770 0.008 Uniaxial (−) 4.00
Rhodolite Garnet 1.740 1.770 — SR 3.84
Ruby 1.762 1.770 0.008 Uniaxial (−) 4.00
Sapphire 1.762 1.770 0.008 Uniaxial (−) 4.00
Star Ruby 1.762 1.770 0.008 Uniaxial (−) 4.00
Star Sapphire 1.762 1.770 0.008 Uniaxial (−) 4.00
Synthetic Color-Change Corundum 1.762 1.770 0.008 Uniaxial (−) 4.00
White Sapphire 1.762 1.770 0.008 Uniaxial (−) 4.00
Yellow Sapphire 1.762 1.770 0.008 Uniaxial (−) 4.00
Color-Change Garnet 1.740 1.765 — SR 3.72
Hessonite Garnet 1.730 1.760 — SR 3.61
Pyrope Garnet 1.730 1.760 — SR 3.78
Tsavorite Garnet 1.730 1.760 — SR 3.61
Grossular Garnet 1.734 1.759 — SR 3.61
Alexandrite 1.746 1.755 0.009 Biaxial (+) 3.73
Cat's Eye Chrysoberyl 1.746 1.755 0.009 Biaxial (+) 3.73
Chrysoberyl 1.746 1.755 0.009 Biaxial (+) 3.73
Diaspore 1.702 1.752 0.048 Biaxial (+) 3.35
Rhodonite 1.716 1.752 0.014 Biaxial (+) 3.60
Spinel 1.712 1.736 — SR 3.60
Kyanite 1.712 1.734 0.016 Biaxial (−) 3.68
Black Spinel 1.712 1.730 — SR 3.60
Chrome Diopside 1.664 1.730 0.030 Biaxial (+) 3.30
Synthetic Spinel 1.725 1.728 — SR 3.63
Taaffeite 1.719 1.723 0.004 Uniaxial (−) 3.61
Star Diopside 1.664 1.721 0.030 Biaxial (+) 3.29
Sinhalite 1.665 1.712 0.038 Biaxial (−) 3.48
Thulite 1.691 1.704 0.009 Biaxial (+) 3.20
Tanzanite 1.691 1.700 0.009 Biaxial (+) 3.35
Kornerupine 1.660 1.699 0.013 Biaxial (−) 3.33
Pearl 1.520 1.690 — Aggregate 2.74
Peridot 1.654 1.690 0.036 Biaxial (+) 3.34
Jadeite 1.654 1.688 — Aggregate 3.34
Jet 1.640 1.680 — SR 1.32
Hiddenite 1.660 1.676 0.016 Biaxial (+) 3.18
Kunzite 1.660 1.676 0.016 Biaxial (+) 3.18
Clinohumite 1.625 1.675 0.028 Biaxial (+) 3.21
Enstatite 1.663 1.673 0.010 Biaxial (+) 3.27
Lapis Lazuli 1.500 1.670 — Aggregate 2.75
Phenakite 1.654 1.670 0.016 Uniaxial (+) 2.96
Prehnite 1.611 1.665 0.030 Biaxial (+) 2.88
Chrome Tourmaline 1.624 1.660 0.018 Uniaxial (−) 3.10
Calcite 1.486 1.658 0.172 Uniaxial (−) 2.71
Coral 1.486 1.658 — Aggregate 2.68
Green Tourmaline 1.616 1.650 0.018 Uniaxial (−) 3.06
Turquoise 1.610 1.650 — Aggregate 2.76
Watermelon Tourmaline 1.616 1.650 0.018 Uniaxial (−) 3.06
Andalusite 1.629 1.649 0.010 Biaxial (−) 3.17
Apatite 1.628 1.649 0.006 Uniaxial (−) 3.20
Indicolite 1.624 1.644 0.018 Uniaxial (−) 3.06
Paraiba Tourmaline 1.624 1.644 0.018 Uniaxial (−) 3.06
Rubellite 1.624 1.644 0.018 Uniaxial (−) 3.06
Tourmaline 1.624 1.644 0.020 Uniaxial (−) 3.06
Topaz 1.609 1.643 0.008 Biaxial (+) 3.53
Larimar 1.595 1.640 0.040 Biaxial (+) 2.80
Danburite 1.627 1.636 0.006 Biaxial (−) 3.00
Chrysocolla 1.575 1.635 — Aggregate 2.20
Nephrite 1.600 1.627 — Aggregate 2.95
Grandidierite 1.590 1.623 0.033 Biaxial (−) 2.95
Sugilite 1.607 1.610 0.002 Uniaxial (−) 2.76
Howlite 1.583 1.608 — Aggregate 2.53
Emerald 1.565 1.602 0.006 Uniaxial (−) 2.72
Morganite 1.572 1.600 0.008 Uniaxial (−) 2.82
Variscite 1.560 1.594 0.031 Biaxial (−) 2.52
Aquamarine 1.577 1.583 0.006 Uniaxial (−) 2.72
Goshenite 1.577 1.583 0.006 Uniaxial (−) 2.72
Heliodor 1.568 1.580 0.006 Uniaxial (−) 2.72
Red Beryl 1.564 1.574 0.007 Uniaxial (−) 2.68
Labradorite 1.559 1.573 0.011 Biaxial (+) 2.70
Rainbow Moonstone 1.559 1.572 0.011 Biaxial (−) 2.70
Scapolite 1.540 1.572 0.020 Uniaxial (−) 2.63
Sunstone 1.537 1.572 0.010 Biaxial (−) 2.63
Serpentine 1.560 1.571 — Aggregate 2.55
Iolite 1.540 1.560 0.012 Biaxial (−) 2.59
Charoite 1.550 1.559 — Aggregate 2.60
Amethyst 1.544 1.553 0.009 Uniaxial (+) 2.65
Ametrine 1.544 1.553 0.009 Uniaxial (+) 2.65
Aventurine 1.544 1.553 — Aggregate 2.65
Citrine 1.544 1.553 0.009 Uniaxial (+) 2.65
Hawk's Eye 1.544 1.553 — Aggregate 2.65
Prasiolite 1.544 1.553 0.009 Uniaxial (+) 2.65
Rock Crystal 1.544 1.553 0.009 Uniaxial (+) 2.65
Rose Quartz 1.544 1.553 0.009 Uniaxial (+) 2.65
Smoky Quartz 1.544 1.553 0.009 Uniaxial (+) 2.65
Tiger's Eye 1.544 1.553 — Aggregate 2.65
Amber 1.539 1.545 — SR 1.08
Agate 1.530 1.543 — Aggregate 2.61
Bloodstone 1.530 1.543 — Aggregate 2.61
Carnelian 1.530 1.543 — Aggregate 2.61
Chalcedony 1.530 1.543 — Aggregate 2.61
Chrysoprase 1.530 1.543 — Aggregate 2.61
Onyx 1.530 1.543 — Aggregate 2.61
Chrysocolla Chalcedony 1.530 1.543 — Aggregate 2.61
Ivory 1.535 1.540 — Aggregate 1.85
Pietersite 1.534 1.540 — Aggregate 2.61
Amazonite 1.514 1.530 0.008 Biaxial (−) 2.56
Moonstone 1.518 1.526 0.008 Biaxial (−) 2.58
Moldavite 1.480 1.510 — SR 2.35
Obsidian 1.480 1.510 — SR 2.40
Hauyne 1.494 1.509 — SR 2.47
Hackmanite 1.483 1.487 — SR 2.27
Sodalite 1.483 1.487 — Aggregate 2.27
Fire Opal 1.370 1.470 — SR 2.00
Opal 1.370 1.470 — SR 2.15
Black Opal 1.440 1.460 — SR 2.10
Fluorite 1.432 1.436 — SR 3.18

Which gemstone has the highest refractive index?
Among the 132 gems in this reference, Cinnabar has the highest refractive index at 2.905–3.256. For comparison, diamond measures 2.417. A standard refractometer reads over the limit (OTL) above roughly 1.81, so high-RI stones are identified by their other properties.

What is the refractive index of diamond?
Diamond's refractive index is 2.417. It is singly refractive with no birefringence — one of the separations from moissanite, which is doubly refractive with visible facet doubling.

What is the refractive index of ruby?
Ruby has a refractive index of 1.762–1.770 with birefringence of 0.008. Sapphire is the same mineral (corundum) and reads the same range.

All instrument charts: Specific Gravity  ·  Mohs Hardness  ·  Birefringence  ·  Fluorescence  ·  Pleochroism  ·  Spectroscope  ·  Optic Character  ·  Chelsea & Hanneman Filters  ·  Dispersion  ·  Magnetism

See also: Full Reference Database  ·  Exam Study Guides  ·  Open dataset (CSV & JSON, CC BY 4.0)

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