How to Identify Natural Color in Tanzanite

As noted in last month's Jewel of the Month (see "Natural-Color Tanzanite," JCK, February 2005, p. 122), natural-color tanzanite can be identified using a standard dichroscope or a Polaroid plate. Tanzanite is a doubly refractive gem, a mineral that splits a light beam in two. Within each beam are two vibration directions, each of which can carry a different color. This is called pleochroism. Looking for pleochroic colors can help identify a number of gemological properties. First, seeing two different pleochroic colors proves that the gem is doubly refractive. This eliminates gems that are singly refractive, like spinel, garnet, and diamond. Doubly refractive gems have either one or two optical axes. A gem that shows three colors (as opposed to two) is biaxial (two axes as opposed to one, called uniaxial). Tanzanite is biaxial. Seeing three different colors in tanzanite strongly i
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