Cat's Eye Gems

In gemology, the term phenomenon (or phenomenal gem) refers to an unusual optical effect that is displayed by a gemstone. Examples include asterism (the star effect), chatoyancy (the cat's eye effect), play-of-color, labradorescence, adularescence, iridescence and color change.

Natural cat's eye
Natural Cat's Eye

The phenomenon of chatoyancy is related to asterism, because both the cat's eye and star effects are caused by light reflecting from inclusions in the gem. The inclusions are typically needles, channels or parallel fibers. When the gem is cut as a cabochon with the base in parallel with the fibers, a pattern resembling the slit eye of a cat may be displayed. When the gem is rotated, the cat's eye appears to glide over the surface of the stone.The chatoyant effect is similar to asterism, except there is one straight ray instead of four or six. Occasionally one will find a cat's eye with two parallel rays.

In the gem trade the term cat's eye is used to refer specifically to cat's eye chrysoberyl, which is regarded as the quintessential cat's eye gem. Cat's eye chrysoberyl is a very hard (8.5 on the Mohs scale) and durable gem that displays a very sharp and well defined cat's eye. It can be found in several colors, including gold and green.

Chrysoberyl Cat's Eye Ceylon 5.33 cts
 Chrysoberyl Cat's Eye

But there are in fact many gem varieties which can exhibit the cat's eye effect. They include cat's eye alexandrite (a type of chrysoberyl that exhibits a color change), opalite cat's eye, cat's eye apatite, tourmaline cat's eye, kornerupine cat's eye and aquamarine cat's eye. 

There are also a number of quartz varieties that can display chatoyancy. These include cat's eye ctrine, quartz cat's eye and the distinctive tiger's eye.  Tiger's eye is a golden and brown material that was first formed as the fibrous blue mineral called crocidolite, which is made up of iron and sodium. The crocidolite was gradually transformed into quartz while maintaining its unique fibrous patterns.

Cat's eye gems are valued according to the distinctness of the cat's eye, the body color, transparency, hardness and rarity. Some cat's eye gems are inexpensive, such as the quartz and apatite varieties. Cat's eye chrysoberyl is traditionally the most valuable and expensive.