Gemstone Refractive Index: The Complete RI and Birefringence Chart

A standard gemological refractometer cannot read a diamond. The contact liquid that couples the stone to the instrument stops working somewhere around a refractive index of 1.80, and every gem above that line, diamond, zircon, sphalerite, demantoid garnet, cassiterite, comes back marked only as over the limit. The first ten rows of the table below sit in that territory. Everything beneath them can be measured in seconds, which is why refractive index remains the fastest identification tool in the laboratory.

Refractive index, usually abbreviated RI, is the ratio between the speed of light in a vacuum and its speed inside a transparent material. Light changes direction when it crosses that boundary, an effect familiar from the way a stick standing in water appears to bend at the surface. Every gem species bends light by an amount fixed by its chemistry and crystal structure, and that number does not change with color, size, or origin.

A higher refractive index means light entering the stone is bent more sharply, so more of it reflects back out through the crown to the eye instead of passing straight through. This is brilliance, the return of white light. It is worth separating from fire, the flashes of spectral color, which comes from dispersion, a different property entirely. Sphalerite illustrates the gap: its refractive index of 2.37 sits below diamond, while its dispersion of 0.156 is more than three times higher.

Most transparent gems split entering light into two rays that travel at different speeds, a property called birefringence or double refraction. Uniaxial minerals such as ruby, sapphire, and tourmaline have two principal indices. Biaxial minerals such as topaz, tanzanite, and peridot have three, and the table lists the lowest and highest of them, since the difference between those two values is what gemologists record. In zircon and sphene the split is wide enough to see doubled facet edges through the crown with a loupe.

Some gems are singly refractive and return a single reading. This group includes diamond, spinel, and all the garnets, along with other cubic crystals like cuprite and fluorite, and amorphous materials with no crystal structure at all, such as opal, amber, and glass. A single reading is itself diagnostic: it rules out every uniaxial and biaxial species in one measurement.

Refractive index rarely identifies a stone alone. Paired with specific gravity, it narrows almost any transparent gem to one species, which is why both values appear together near the top of a laboratory report. Our guide to reading a gem certificate explains where to find them and what the surrounding entries mean.

Gem names shown as links lead to our current selection of that stone.

	Gem Variety	Refractive Index	Double Refraction
	Hematite	2.940-3.220	0.287
	Cinnabar	2.905-3.256	0.351
	Cuprite	2.849	none
	Diamond	2.417-2.419	none
	Sphalerite	2.368-2.371	none
	Cassiterite	1.997-2.098	0.096-0.098
	Scheelite	1.918-1.937	0.010-0.018
	Demantoid Garnet	1.88-1.94	none
	Uvarovite Garnet	1.865	none
	Sphene (Titanite)	1.843-2.110	0.100-0.192
	Zircon	1.810-2.024	0.002-0.059
	Cerussite	1.804-2.079	0.274
	Spessartite Garnet	1.790-1.820	none
	Painite	1.787-1.816	0.029
	Almandine Garnet	1.770-1.820	none
	Ruby	1.762-1.778	0.008
	Star Ruby	1.762-1.778	0.008
	Sapphire	1.762-1.778	0.008
	Star Sapphire	1.762-1.778	0.008
	Mali Garnet	1.760-1.780	none
	Benitoite	1.757-1.804	0.047
	Alexandrite	1.746-1.763	0.007-0.011
	Alexandrite Cat's Eye	1.746-1.763	0.007-0.011
	Chrysoberyl	1.746-1.763	0.007-0.011
	Chrysoberyl Cat's Eye	1.746-1.763	0.007-0.011
	Vanadium Chrysoberyl	1.746-1.763	0.007-0.011
	Rhodolite Garnet	1.742-1.785	none
	Color-Change Garnet	1.742-1.780	none
	Malaia Garnet	1.742-1.780	none
	Grossular Garnet	1.734-1.759	none
	Tsavorite Garnet	1.734-1.759	none
	Leuco Garnet	1.734-1.759	none
	Hessonite Garnet	1.730-1.757	none
	Epidote	1.729-1.768	0.015-0.049
	Bastnasite	1.720-1.820	0.100
	Azurite	1.720-1.848	0.108-0.110
	Pyrope Garnet	1.720-1.756	none
	Taaffeite	1.719-1.730	0.004-0.009
	Rhodonite	1.716-1.752	0.010-0.014
	Spinel	1.712-1.782	none
	Kyanite	1.710-1.734	0.015-0.033
	Diaspore	1.702-1.750	0.048
	Serendibite	1.701-1.743	0.005
	Idocrase (Vesuvianite)	1.700-1.723	0.002-0.012
	Tanzanite	1.691-1.700	0.009
	Zoisite	1.691-1.700	0.009
	Dumortierite	1.678-1.689	0.015-0.037
	Sinhalite	1.665-1.712	0.036-0.042
	Chrome Diopside	1.664-1.730	0.024-0.031
	Diopside	1.664-1.730	0.024-0.031
	Kornerupine	1.660-1.699	0.012-0.017
	Hiddenite	1.660-1.681	0.014-0.016
	Kunzite	1.660-1.681	0.014-0.016
	Spodumene	1.660-1.681	0.014-0.016
	Axinite	1.656-1.704	0.010-0.012
	Malachite	1.666-1.909	0.254
	Sillimanite	1.655-1.684	0.014-0.021
	Jadeite Jade	1.652-1.688	0.020
	Peridot	1.650-1.703	0.036-0.038
	Enstatite	1.650-1.680	0.009-0.012
	Euclase	1.650-1.677	0.019-0.025
	Phenakite	1.650-1.670	0.016
	Dioptase	1.644-1.709	0.051-0.053
	Jet	1.640-1.680	none
	Jeremejevite	1.637-1.653	0.007-0.013
	Barite	1.636-1.648	0.012
	Danburite	1.630-1.636	0.006-0.008
	Clinohumite	1.629-1.674	0.028-0.041
	Apatite	1.628-1.649	0.002-0.006
	Cat's Eye Apatite	1.628-1.649	0.002-0.006
	Andalusite	1.627-1.649	0.007-0.013
	Smithsonite	1.621-1.849	0.228
	Datolite	1.621-1.675	0.040-0.060
	Celestine	1.619-1.635	0.010-0.012
	Chrome Tourmaline	1.614-1.666	0.014-0.032
	Paraiba Tourmaline	1.614-1.666	0.014-0.032
	Rubellite Tourmaline	1.614-1.666	0.014-0.032
	Tourmaline	1.614-1.666	0.014-0.032
	Actinolite	1.614-1.653	0.020-0.025
	Hemimorphite	1.614-1.638	0.022
	Prehnite	1.611-1.669	0.021-0.039
	Gaspeite	1.610-1.810	0.22
	Turquoise	1.610-1.650	0.040
	Topaz	1.609-1.643	0.009-0.016
	Imperial Topaz	1.609-1.643	0.009-0.016
	Sugilite	1.607-1.611	0.001-0.004
	Brazilianite	1.602-1.623	0.019-0.021
	Pezzotaite	1.601-1.620	0.008-0.011
	Rhodochrosite	1.600-1.820	0.208-0.220
	Nephrite Jade	1.600-1.627	0.027
	Larimar	1.595-1.645	0.038
	Eudialyte	1.591-1.633	0.003-0.010
	Grandidierite	1.590-1.623	0.033
	Howlite	1.586-1.605	0.019
	Amblygonite	1.578-1.646	0.024-0.030
	Emerald	1.565-1.602	0.005
	Aquamarine	1.565-1.602	0.005
	Morganite	1.562-1.605	0.004-0.010
	Goshenite Beryl	1.562-1.605	0.004-0.010
	Golden Beryl	1.562-1.605	0.004-0.010
	Red Beryl (Bixbite)	1.562-1.605	0.004-0.010
	Labradorite	1.559-1.570	0.008-0.010
	Hambergite	1.553-1.628	0.072
	Charoite	1.550-1.561	0.004-0.009
	Amethyst	1.544-1.553	0.009
	Aventurine	1.544-1.553	0.009
	Ametrine	1.544-1.553	0.009
	Citrine	1.544-1.553	0.009
	Gem Silica	1.544-1.553	0.009
	Rock Crystal	1.544-1.553	0.009
	Rose Quartz	1.544-1.553	0.009
	Smoky Quartz	1.544-1.553	0.009
	Andesine	1.543-1.551	0.009
	Iolite	1.542-1.578	0.008-0.012
	Scapolite	1.540-1.579	0.009-0.037
	Jasper	1.54	none
	Amber	1.539-1.545	none
	Oligoclase	1.538-1.550	0.007-0.013
	Aragonite	1.530-1.685	0.155
	Williamsite	1.530-1.575	0.001-0.002
	Agate	1.530-1.540	0.004-0.009
	Bloodstone	1.530-1.540	0.004-0.009
	Chalcedony	1.530-1.540	0.004-0.009
	Chrome Chalcedony	1.530-1.540	0.004-0.009
	Carnelian	1.530-1.540	0.004-0.009
	Chrysoprase	1.530-1.540	0.004-0.009
	Onyx	1.530-1.540	0.004-0.009
	Lepidolite	1.525-1.587	0.013
	Sunstone	1.525-1.548	0.010
	Amazonite	1.522-1.530	0.008
	Pearl	1.52-1.69	0.158
	Ammolite (Korite)	1.52-1.68	0.155
	Orthoclase	1.518-1.530	0.008
	Moonstone	1.518-1.528	0.008
	Carletonite	1.517-1.521	0.004
	Poudretteite	1.511-1.532	0.021
	Petalite	1.502-1.518	0.012-0.017
	Lapis Lazuli	1.50	none
	Hauyne	1.498-1.510	none
	Ulexite	1.491-1.520	0.029
	Calcite	1.486-1.658	0.172
	Cobaltoalcite	1.486-1.658	0.172
	Coral	1.486-1.658	0.160-0.172
	Moldavite	1.48-1.54	none
	Hackmanite	1.48	none
	Sodalite	1.48	none
	Chrysocolla	1.46-1.57	0.023-0.040
	Obsidian	1.45-1.55	none
	Fluorite	1.434	none
	Mexican Fire Opal	1.37-1.52	none
	Opal	1.37-1.52	none

Frequently Asked Questions

What is refractive index in a gemstone?

Refractive index is the ratio between the speed of light in a vacuum and its speed inside the stone. It is fixed by the mineral's chemistry and crystal structure, so it does not change with the stone's color, size, cut, or origin. That constancy is what makes it a reliable identification tool.

Which gemstone has the highest refractive index?

Among the materials listed here, cinnabar and hematite reach the highest values, above 2.9. Among gems commonly cut for jewelry, diamond leads at 2.417 to 2.419, followed by sphalerite at roughly 2.37 and zircon, which reaches 2.024.

What does over the limit mean on a gem report?

A standard refractometer relies on a contact liquid that stops functioning at a refractive index near 1.80. Any stone above that value gives no reading and is recorded as over the limit. Diamond, zircon, sphalerite, demantoid garnet, and cassiterite all fall into this group, and the laboratory identifies them by other means.

What is the difference between refractive index and dispersion?

Refractive index governs brilliance, the return of white light through the crown. Dispersion governs fire, the flashes of spectral color. They are separate properties and do not rise together: sphalerite has a lower refractive index than diamond and a dispersion of 0.156, more than three times higher.

What is double refraction in a gemstone?

Most transparent gems split entering light into two rays travelling at different speeds, and the difference between the two refractive indices is recorded as double refraction, also called birefringence. Where the value is large, as in zircon and sphene, facet edges appear doubled when viewed through the crown with a loupe.

Can a refractometer tell a natural gemstone from a lab-grown one?

No. A laboratory-grown ruby has the same chemistry and crystal structure as a natural one, so it returns the same refractive index. Separating them relies on inclusions, growth structures, and spectroscopy, which is why a report from a recognized laboratory such as GIA, SSEF, or ČGL matters more than any single measurement.

How is refractive index measured?

The stone is placed on the glass hemisphere of a refractometer with a drop of high-index contact liquid between them, and the reading is taken from a scale under monochromatic light. A single facet and a few seconds are enough, which is why it is usually the first test performed.

Does refractive index affect a gemstone's value?

Only indirectly. A high refractive index contributes to brilliance, which makes a well-cut stone livelier, but value is driven by color, clarity, size, treatment, and origin. Fluorite has a refractive index close to that of quartz and neither figure explains their prices.