... The RHG-181, commonly known as Gemstone Refractometer, is the primary and most powerful tool in ... to its chemical composition and crystallization structure, each gem has a typical refractive index. In fact, ... Through these parameters, you can determine whether the gem is a Single Refraction (Isotropic) or Double ... etc.)- User's Manual RHG-181 Gem Refractometer w/ Built-in Light Source + RI Oil