This device is used to determine a gemstone’s refractive index. The refractive index (RI) of a material is the ratio of the speed of light in the air to the speed of light in the material.
A gem’s brightness is directly linked to its refractive index. The higher the index, the more brightly gem shines.
The gem’s birefringence can be determined from the maximum difference in IR according to the direction of light.
The refractometer plays a major part in classical gemmology, as in addition to IR and birefringence, it helps determine other data such as the number of optical axes in the gem and the optical gem sign.