Birefringence

A refractometer is a tool gemologists use to measure a gem's double refraction, known as its birefringence.
Photo courtesy of Scott Papper, GIA GG, AJP, Global Gemology & Appraisals
Doubly refractive gems split light into 2, and sometimes 3 directions once it enters the gem.

Birefringence is the measure of a gem's double refraction.

Some gems, like ekanite, have a very small birefringence of 0.001 while other gems, such as tourmaline, are known to have high birefringences of 0.018 to 0.020.