Refractive Index of Gemstones — Complete Reference Chart

RI values for 130 gemstone species, sorted high to low by maximum RI. Includes birefringence and optic character.

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 covers the full range encountered in gemology, from low-RI amber (1.54) and fluorite (1.434) up to high-RI cassiterite (2.00–2.10) and demantoid garnet (1.880–1.889).

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
Sphalerite 2.369 2.500 — SR 4.09
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
Cubic Zirconia 2.150 2.180 — SR 5.80
Sphene 1.843 2.110 0.100 Biaxial (+) 3.53
Cassiterite 2.010 2.100 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 1.685 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 3.61
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
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 1.460 1.630 — 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 1.590 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
Charoite 1.550 1.559 0.009 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
Iolite 1.542 1.551 0.009 Biaxial (−) 2.59
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
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

See also: Full Reference Database  ·  Specific Gravity Chart  ·  UV Fluorescence Chart  ·  Mohs Hardness Chart

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