Refractive Index
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Definition
A numerical value expressing how much a gemstone bends light as it passes through the stone, calculated as the ratio of the speed of light in a vacuum to the speed of light in the gem.
Detailed Explanation
The refractive index (RI) is one of the most diagnostic properties used to identify gemstones. Diamond has an RI of 2.42, while quartz measures 1.54–1.55. A refractometer is the standard tool used to measure RI in the gem trade, and most transparent gemstones have characteristic RI ranges that distinguish them from simulants and look-alikes.