Already measured your stone's refractive index? Enter it below — add birefringence too, if you have it — and see which gemstone species match.
The single RI value (or the higher of two shadow edges) read off your refractometer.
The difference between the two shadow edges. Leave the box above unchecked if you only saw one edge (singly refractive).
This tool suggests likely candidates based on typical published RI & birefringence ranges — it isn't a substitute for full gemological testing (specific gravity, pleochroism, spectroscopy, inclusions) for identifying or valuing a specific stone.
Refractive index (RI) measures how much light bends when it enters a gemstone — and like specific gravity, it's a near-constant fingerprint of the species. Diamond reads 2.417, sapphire 1.762–1.770, quartz just 1.544–1.553. Two look-alike stones rarely share an RI, which is why the refractometer is the first instrument a gemologist reaches for.
Refractive index measures how much light slows and bends when it passes from air into the stone. Each species has a characteristic range — diamond 2.417, ruby/sapphire 1.762–1.770, emerald 1.577–1.583 — which is why a single refractometer reading is one of the fastest ways to narrow down an unknown gemstone.
Most gemstones bend light differently depending on direction. A doubly refractive stone shows two shadow edges on a refractometer, and the difference between them is the birefringence. It's a powerful separator: aquamarine and topaz can share a similar RI, but topaz's birefringence of about 0.009 vs. aquamarine's 0.006 — plus different optic character — tells them apart. Singly refractive stones like diamond and spinel show only one edge.
Diamond's refractive index is 2.417 — among the highest of all common gemstones, which is a big part of why it sparkles so intensely. Only a few materials read higher, such as zircon (up to 1.985) and sphene (up to 2.11). An RI of 2.417 plus SG around 3.52 is strong evidence of diamond.
RI narrows an unknown stone to a short list of candidates, but it rarely proves identity on its own. Confirm with at least one independent test — specific gravity, hardness, pleochroism, or inclusions — before buying, selling, or certifying a stone.
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