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Frequently Asked Questions (FAQ) / Birefringence of Gemstones

What does "birefringence" refer to? I don't know if my gem has it.

Birefringence is a measurement of the difference between the two refractive indices in gems that are doubly refractive, and it ranges from a low of .003 to a high of .287. When a beam of light enters a doubly refractive gem, it is split into two beams, each travelling at a different speed and on a different path through the crystal. Relatively few gemstones are singly refractive: they have only one refractive index. Most gemstones are doubly refractive; they have two different refractive indices. The best-known gems with that property are diamond, spinel and garnet. These optical properties are very useful tools in gem identification.

For more information see our article on singly refractive gems.

Return to FAQ HOME

  • First Published: February-20-2006
  • Last Updated: March-22-2017
  • © 2005-2018 GemSelect.com all rights reserved.
    Reproduction (text or graphics) without the express written consent of GemSelect.com (SETT Company Ltd.) is strictly prohibited.
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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