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Frequently Asked Questions (FAQ) / Refractive Index of a Gemstone

What is the significance of the refractive index of a gemstone? Is a higher value better?

Refractive index, or RI, is the measurement of the bending of light when it passes from air to another material. If light passes from air into a transparent material at one angle, it is deflected at a different angle (the coincident angle). Gemstones with a higher RI are generally more brilliant than those with a low RI, because more light is returned back out of the top or crown of the stone, instead of passing through the bottom or pavilion. Diamond has an RI of about 2.4; quartz, about 1.54 to 1.55. The RI of most gemstones is easily measured using a simple optical instrument known as a refractometer.

See our refractive index chart for ratings of the different gem varieties.

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  • First Published: February-20-2006
  • Last Updated: November-11-2014
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Size and Weight

Gems are always measured in Millimeter (mm)

Dimensions are given as;
length x width x depth,
except for round stones which are;
diameter x depth

Select gems by size, not by weight!
Gem varieties vary in density, so carat weight is not a good indication of size

Note: 1ct = 0.2g

Size Comparison Chart