A gem refractometer is an essential tool for gemstone identification, used in gemological laboratories and by jewelers. With a gemstone refractometer, the refractive index of gemstones can be precisely measured, providing valuable information for their identification. The working principle of a gem refractometer is based on light entering through a window on the back and being projected onto the stone. A hinged lid allows a gemstone to be placed on the flat side of a half cylinder made of high-purity special glass. The front ocular magnifies the scale. The lens is equipped with a rotatable polarizing filter. Both the filter and the lens are removable.

The shadow of the gem is cast on the scale and the RI is read where the shadow line runs.