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.