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 indicates natural color, since heat treatment is specifically used to eliminate the yellow/green/brown color direction, creating yet another blue pleochroic color.
To see a pleochroic color, one needs to separate the two or three colors. You can do that using either a Polaroid plate (which eliminates one color) or a transparent doubly refractive material, such as calcite (which separates the two). At the most, only two colors can be seen in any one direction.
The following images show natural-color zoisites; greens, peach, and tanzanite purplish-blues. The tanzanite shots were taken using a standard Polaroid plate in front of the gem, between the gem and the camera. The jeweler-gemologist can easily do the same. If you do not have access to gemological equipment, you can use one lens of your Polaroid sunglasses.