Refractive Index of Gemstones — Complete Reference Chart

Last updated: April 2026

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).

Studying for the GIA or FGA practical? GemID drills refractometer readings and runs timed mock exams built around the real exam formats.

Start practicing free →
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  ·  Exam Study Guides

Enter your RI reading in GemID and the engine narrows your candidates automatically.

Try GemID Free →