Refractive Index by Color — Gem Elimination Chart

Elimination is how practical gem identification actually works: no single observation names a stone, but each observation removes candidates. Body color is the first cut — it costs nothing to observe and immediately reduces 130 species to a smaller set. A refractometer reading is the second cut, and it is the sharpest one: keep only the species whose published RI range contains your reading, and the candidate list usually drops to a handful.

The tables below make that second cut mechanical. Find your stone’s color section, run a finger down the RI column to your reading, and the surviving rows are the candidates consistent with what you have observed so far. From there, the SG and optic-character columns tell you which follow-up test separates the survivors — two candidates with near-identical RI rarely share both a specific gravity range and an optic character. For full sorted tables of those properties, see the refractive index chart and the specific gravity chart.

Two cautions. First, a reading near the edge of a species’ range deserves a neighbor check — published ranges are boundaries, not walls, and instrument calibration adds its own tolerance. Second, an elimination table brackets candidates; it does not identify. A stone that survives every cut here is consistent with the remaining species, and confirming it takes the follow-up tests — or a lab. This is the same elimination logic the three practical exams reward, worked end-to-end in the worked practice exam.

Can a refractive index reading alone identify a gemstone?
No. Many species overlap on the refractometer — a single reading is consistent with several candidates, which is why this sheet groups the 130 species by body color first. Color narrows the field, the RI reading cuts it down, and specific gravity and optic character separate what remains. Treat every match as a candidate, not a conclusion.

How do I use this elimination sheet?
Start from the stone’s body color and go to that section. Take one refractometer reading, then keep only the rows whose RI range contains it — those species are the candidates consistent with your observations. Confirm with a hydrostatic SG measurement and a polariscope optic-character call from the remaining columns.

Why do some gems appear in more than one color section?
Many species occur in several body colors — sapphire, tourmaline, and spinel span multiple categories. Each species is listed in every color category recorded for it in the GemID database, so whichever color you start from, the candidate list is complete for that color.