... stone. Due to its chemical composition and crystallization structure, each gem has a typical refractive index. In fact, you can also ... these parameters, you can determine whether the gem is a Single Refraction (Isotropic) or Double Refraction (Anisotropic or Birefringence).Key Features: 1 ... .e. Calcite, Jadeite, Quartz etc.)- User's Manual RHG-181 Gem Refractometer w/ Built-in Light Source + RI Oil