REFRACTIVE INDEX
Refractive Index, Birefringence, And The Refractometer
Light that enters a gemstone is refracted (bent) from its original path in air as it enters the denser medium. Cubic and non-crystalline materials are singly refractive, which means that the light is refracted equally in all directions. Gemstones from the other crystal systems are doubly refractive. In doubly refractive gemstones, the light entering the gem is split into two rays of light; each is slowed and refracted by a different amount. Where the difference is large, such as in calcite, double refraction can be seen as a double image through the gemstone. In sphalerite, a doubling of the image of the back facets (pavilion facets) can be seen when viewing them through the front of the stone (crown facets).
There is a mathematical relationship between the angle at which light strikes a gemstone and the angle of refraction, from which the refractive index (Rl) of a gemstone can be calculated. The refractive index of most gems can be measured accurately using a refractometer or Brewster Angle Meter and can be used to help identify the stone. Singly refractive gemstones have a single refractive index. Doubly refractive gemstones have a range of refractive indices; the difference between the maximum and minimum values is the birefringence (DR.).
