What does "birefringence" refer to? I don't know if my gem has it.

Birefringence is a measurement of the difference between the two refractive indices in gems that are doubly refractive, and it ranges from a low of .003 to a high of .287. When a beam of light enters a doubly refractive gem, it is split into two beams, each travelling at a different speed and on a different path through the crystal. Relatively few gemstones are singly refractive: they have only one refractive index. Most gemstones are doubly refractive; they have two different refractive indices. The best-known gems with that property are diamond, spinel and garnet. These optical properties are very useful tools in gem identification.

For more information see our article on singly refractive gems.

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