Refractive index (RI) measures how much a gem slows and bends light as it enters the stone. It is one of the most useful numbers in gem identification because it stays nearly constant for each mineral and can be read without harming the stone. Diamond has an RI of about 2.42, quartz about 1.54 to 1.55, and most other gems fall somewhere between 1.4 and 2.0.
What refractive index measures
Light travels fastest in a vacuum. When it passes into a denser material such as water, glass or a gemstone, it slows down. Refractive index is the ratio of light's speed in a vacuum to its speed in the material. An RI of 1.50 means light moves 1.5 times slower inside the stone than in a vacuum. Air is so close to a vacuum (about 1.0003) that gemologists treat the two as the same.
When light strikes a gem at an angle, the change in speed bends its path. A spoon in a glass of water looks broken at the surface for the same reason. The higher the RI, the more strongly light bends.
Snell's law in plain terms
Snell's law describes how much light bends when it crosses from one material to another. In plain terms: the sharper the slowdown, the more the ray tilts toward a line drawn straight into the surface. Going the other way, from a gem out into air, the ray tilts away from that line.
Past a certain angle, light leaving the stone cannot escape at all and reflects back inside. This is total internal reflection, and the angle at which it starts is smaller in high-RI gems. That is why a well-cut diamond returns so much light: rays bounce off the pavilion facets instead of leaking out the back. The diamond optical properties page goes into the details, and the gemstone optics page covers brilliance and fire in other stones.
How a refractometer works
The gem refractometer uses total internal reflection to read RI directly. Inside the instrument is a hemisphere or prism of high-RI glass. A gemologist places a drop of contact liquid on the glass and sets a flat, polished facet of the gem on it, usually the table. Light enters through the side, and where the angle passes the reflection threshold for that gem, a boundary forms between light and dark areas. Looking through the eyepiece, the user sees a shadow edge against a printed scale and reads the RI where the edge falls.
Most readings use a yellow light source near 589 nanometers (the sodium line) because RI changes slightly with wavelength. Readings are usually given to two or three decimal places.
The 1.81 limit
Standard refractometers have an upper limit near 1.81. Two things set it: the RI of the glass in the instrument, and the RI of the contact liquid. The usual liquid is a saturated solution of sulfur and tetraiodoethylene in diiodomethane, which has an RI of about 1.79 to 1.81. A gem with an RI higher than the liquid gives no edge at all, only a dark field with a faint line at the liquid's own reading. This is called an over-the-limit reading.
The contact liquid is toxic and should be handled with care, cleaned off the stone and the instrument promptly, and kept away from skin.
Gems above the limit include zircon, demantoid garnet, titanite, cubic zirconia, moissanite and diamond. For these, gemologists rely on other tests, such as specific gravity, doubling, dispersion, thermal or electrical testers, or spectroscopy.
RI chart of common gems
| Gem | Refractive index | Birefringence |
|---|---|---|
| Opal | 1.37-1.47 (often about 1.45) | None |
| Fluorite | 1.434 | None |
| Moonstone (orthoclase) | 1.518-1.526 | 0.005-0.008 |
| Quartz (amethyst, citrine) | 1.544-1.553 | 0.009 |
| Chalcedony | 1.53-1.54 | Usually not resolved |
| Iolite | 1.54-1.55 | 0.008-0.012 |
| Beryl (emerald, aquamarine) | 1.57-1.59 | 0.004-0.009 |
| Topaz | 1.61-1.64 | 0.008-0.010 |
| Turquoise | 1.61-1.65 | Spot reading |
| Tourmaline | 1.62-1.64 | 0.014-0.032 |
| Peridot | 1.65-1.69 | 0.035-0.038 |
| Jadeite | About 1.66 | Spot reading |
| Spodumene (kunzite) | 1.66-1.68 | 0.014-0.016 |
| Tanzanite | 1.69-1.70 | 0.008-0.013 |
| Spinel | 1.718 | None |
| Grossular garnet | 1.73-1.75 | None |
| Chrysoberyl | 1.74-1.76 | 0.008-0.010 |
| Pyrope and almandine garnet | 1.72-1.81 | None |
| Corundum (ruby, sapphire) | 1.76-1.77 | 0.008-0.010 |
| Zircon (high) | 1.92-1.98 | Up to 0.059 |
| Cubic zirconia | 2.15-2.18 | None |
| Diamond | 2.417 | None |
| Synthetic moissanite | 2.65-2.69 | 0.043 |
Many gems have a narrow range because their chemistry varies a little from one deposit to the next. Tourmaline and garnet vary more because they are mineral groups, not single species.
Single and double refraction readings
Gems in the cubic crystal system, plus amorphous materials like glass and opal, are singly refractive. Light travels through them at one speed in every direction, so the refractometer shows a single shadow edge that stays put when the stone is turned.
All other crystalline gems are doubly refractive. Light entering them splits into two rays traveling at different speeds, so the refractometer shows two shadow edges. The difference between the highest and lowest readings is the birefringence. By rotating the stone and a polarizing filter on the eyepiece, a gemologist can tell whether the gem is uniaxial (one edge stays fixed while the other moves) or biaxial (both edges can move). The birefringence page explains these optic characters.
Cabochons and carvings have no flat facet. For these, gemologists use the spot method: a tiny drop of liquid on the curved surface, viewed with the eye away from the eyepiece, gives an approximate RI to two decimal places. Polycrystalline materials such as jade and turquoise also give spot or vague readings.
Using RI for identification
RI rarely settles an identification alone, but it narrows the field fast. A red stone reading 1.762 to 1.770 with birefringence of 0.008 is corundum, so it is ruby, either natural or synthetic. A red stone reading 1.718 with no birefringence is spinel. A red stone over the limit with no doubling and high dispersion could be cubic zirconia. Gemologists combine RI with specific gravity, a spectroscope reading and a look under the microscope, as described in the guide on how to identify gemstones. The full set of tools is on the gemological instruments page.
Frequently asked questions
What gem has the highest refractive index?
Among common jewelry stones, synthetic moissanite (about 2.65 to 2.69) is higher than diamond (2.417). Among natural gems, diamond is the highest that is widely set in jewelry.
Why can't a standard refractometer read diamond?
Diamond's RI of 2.42 is above the contact liquid's RI of about 1.79 to 1.81. The instrument shows no shadow edge for any gem above that limit.
Does a higher refractive index mean a more brilliant gem?
A higher RI lets a cutter return more light, so high-RI stones tend to look brighter when well cut. Cut quality, transparency and dispersion all affect the final appearance too.
Can I measure RI at home?
A refractometer is affordable, but the toxic contact liquid and the skill needed to read it correctly make home testing tricky. A trained gemologist gives more reliable results.