What refractive index measures
Light slows down when it enters a gemstone, and the refractive index is the ratio of its speed in air to its speed inside the stone. A refractive index of 1.54 means light travels 1.54 times more slowly in the gem than in air; diamond’s 2.42 means it travels at well under half its speed in air. The slower the light, the more sharply it bends as it crosses the surface — which is where much of the brilliance of a well-cut diamond comes from.
Because the figure depends on the crystal structure and chemistry of the material rather than on its colour, cut or size, it is one of the most useful single numbers in gem identification. A red stone reading 1.762 to 1.770 is behaving like corundum; a red stone giving a single reading near 1.718 is behaving like spinel. Both can look the same to the eye. The instrument can tell them apart in seconds.
Reading a refractometer, and what birefringence adds
A gem refractometer works by placing a polished facet on a glass hemisphere with a drop of contact liquid between them, then reading the shadow edge against a scale through an eyepiece. A singly refractive material — diamond, spinel, garnet, glass — gives one reading that does not move as the stone is rotated. A doubly refractive material splits light into two rays travelling at different speeds, and shows two shadow edges whose positions change as the stone turns.
Birefringence is the difference between the highest and lowest of those readings. It is reported in the table above wherever GIA publishes a usable figure. A peridot, at 0.035 to 0.038, shows a wide gap between its two edges; a ruby, at 0.008 to 0.010, a narrow one. Two gems with overlapping refractive index ranges can often be separated by birefringence alone, and a stone that shows no birefringence at all rules out every doubly refractive candidate on the list.
Cabochons, carvings and aggregates such as jade and turquoise have no flat facet to read, so they are measured by the spot method: a small spot of contact liquid is viewed and the reading taken where the spot changes from light to dark. It gives one approximate figure, not two edges, which is why GIA describes birefringence for jade and turquoise as usually not detectable.
Why overlapping ranges are normal
Many gems share part of their range. Ruby and sapphire are the same mineral, corundum, so their figures are identical. Emerald and aquamarine are both beryl and read the same. Amethyst and citrine are both quartz. The garnet group spans 1.714 to 1.888 because it is a family of related minerals rather than one, so a garnet reading overlaps spinel at one end and corundum in the middle.
That is why the reading box on this page lists every gem whose published range contains a number rather than naming one. A refractometer narrows the field. A gemmologist then adds specific gravity, a polariscope, magnification, a spectroscope and, where it matters, laboratory testing. Treatments and synthetics are a further question: a laboratory-grown ruby has the same refractive index as a natural one, so a matching reading says nothing about origin.
Where the figures come from, and what is left out
Every range here is copied from the facts panel of the GIA Gem Encyclopedia page for that gem, and is shown exactly as published: a range stays a range and a single value stays single. Where GIA adds a tolerance, as it does for the three types of zircon and for both kinds of jade, the tolerance is shown in the note rather than folded into the range.
Two kinds of figure are deliberately missing. Where GIA’s printed birefringence cannot be right as published — a range written from a larger number down to a smaller one, or a tolerance printed in its place — the table shows a dash rather than repeating it or guessing what was meant. And gems without a facts panel in the encyclopedia, including moissanite and cubic zirconia, are not listed, because filling them in from a different source would mix two sets of figures without saying so.