Which instrument is used to measure a gem's refractive index?

• A. Polariscope.
• B. Spectroscope.
• C. Microscope.
• D. Refractometer.

Answer: (D)

Understanding how light slows and bends as it enters a gemstone is the idea behind measuring refractive index, a key property used to identify stones. The instrument designed for this is the refractometer. It measures the angle at which light is refracted as it passes into the gemstone, giving a numerical refractive index value. For most gemstones you get a single RI, while doubly refractive crystals can show two indices depending on crystal orientation, which you can reveal with proper accessories on the refractometer. This measurement is central because many gem species have characteristic RI ranges that help distinguish them and assess authenticity. Other tools have different purposes: a polariscope checks for double refraction and internal stress patterns, a spectroscope analyzes how the stone absorbs light across wavelengths, and a microscope provides magnified views of inclusions and surface features.