The returning of white light to the eye after entering the gem is known as refraction.
The refractive index of a gem measures the amount of refraction, and has a direct connection to the speed of light travelling through the gem. The higher the refractive index, the greater the amount of brilliance and often will result in greater dispersion.
Transparent and translucent gems are normally faceted primarily to display their refraction, whereas opaque gems are normally cabochon cut, to show their surface lustre.
A refractometer is used to measure the level of refraction and its results are one of the more accurate methods for gemmologists to identify different gems.
Completely transparent glass has a refractive index of 1.5. This means that the light travels at 1/1.5 or 0.67 times the speed in air. The higher the refractive index of a gem, the greater its brilliance will be.
