Refractometer

Gem Identification

Definition

A gemological instrument that measures a polished gemstone's refractive index by determining the critical angle at which total internal reflection occurs between the stone and the instrument's reference glass.

Detailed Explanation

The standard gem refractometer uses a dense reference glass hemisphere (RI approx. 1.81) with a monochromatic sodium light source. A tiny drop of contact fluid (methylene iodide or a synthetic equivalent, RI ~1.79) bridges the gem's flat facet to the glass. The operator observes a shadow edge on a scale calibrated in RI values. Most instruments read from 1.35 to 1.81, covering the majority of gem species. Stones above this range (diamond, demantoid, zircon) require approximation methods. The refractometer is considered the single most useful instrument in a gemologist's kit.

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