Refractive Index

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
A gem's refractive index is the single most important piece of information in a gemologist's quest to identify a gem.

R.I. = Velocity of Light in Air ÷ Velocity of Light in a Gemstone

Click Here to view the Refractive Index Chart.