Also, the low-temperature formation of the spicules, as compared to the high temperature stretching process of commercial fibre optics, allows for the addition of impurities which improve the refractive index.
It can also be used as an optical contact liquid, in conjunction with the gemmological refractometer, for determining the refractive index of certain gemstones.
Gemstones are basically categorized based on their crystal structure, specific gravity, refractive index, and other optical properties, such as pleochroism.