Refractive Index is one of the most important optical parameters used to identify and authenticate polished precious and semi-precious gemstones. LPRM-90 Gemological refractometer is a low cost, lightweight measuring instrument used by jewelers and traders of gemstones. The Gemological refractometer can also be used to identify whether a particular stone is Isotropic or Anisotropic (uniaxial or biaxial).