Most gemstones are birefringent, however the optical effect is only noticeable to the naked eye in a few.
Also referred to as double refraction, birefringence is the splitting of a single ray of light into two rays.
Double refraction is often beautiful to observe and is best witnessed in Zircon. As the light splits in a faceted gemstone, it hits the inside of the pavilion facets and bounces around like an image in a hall of mirrors. The effect seen when studying a gem that has birefringence through its table and crown facets is incredibly beautiful. In addition to Zircon, the effect can also be seen in Calcite and Moissanite (a man made item that is sold as a replacement to Diamond).
Birefringent gemstones have two different refractive indices; this makes the optical phenomena very useful for gem dealers to correctly identify certain man-made fakes from real gemstones. In order to measure birefringence, a gem is placed in a refractometer and two readings are taken.
All gemstones that are pleochroic are also birefringent. In fact other than Diamonds, Spinel and members of the Garnet family, most other gemstones featured in this book are all birefringent; however some split the light more than others.
