One of the most important properties used when identifying gemstones is the refractive index, which measures the gem’s ability to bend a ray of light as it enters or leaves. You use a tool called a refractometer to measure, this is a non-destructive way of identifying a gem. A refractometer works best when on a convex, or flat surface. This object can give very accurate readings, from a minimum of 1.30 to 1.81. The higher number of the limit, pertains to the refractive index of a contact liquid. Contact liquid is used to form an optical contact between the gemstone and the hemicylinder of the refractometer. Zircon and some garnets are of the few known to have indices above the normal refractometer’s scale. To reach these higher indices, you can use a refractometer that has a diamond hemicylinder, these can be expensive and are not required for other gems in the range of 1.30 to 1.81.
You’ll find that many gemstones are doubly refractive, meaning two readings can be taken at the same time on one gem. These readings will range different numbers, you can find the highest and lowest indice by rotating the gem around on the hemicylinder. During testing with a refractometer, you can actually check for both the refractive index range, and the birefringence at the same time.