From the French words “chat oeil,” meaning cat’s eye.
The word chatoyancy is used to describe an optical effect that certain gems have, which resemble the opening and closing of eyes.
The phenomenon is commonly called cat’s eye effect, for when the light hits the surface of the polished gemstone, a narrow line of light appears, which looks dramatically similar to that of a cat’s eye. Chatoyancy is an effect caused by tiny fibrous inclusions that are naturally arranged in a parallel configuration.
For a gemstone to show this effect it must be cabochon cut, and its inclusions must run parallel to the base. Examples of gems that can feature chatoyancy are:Tourmaline, Sapphire, Quartz, Chrysoberyl, Fibrolite and of course, Tiger’s Eye.
One word of warning: I spoke to a lady once who told me that her beloved Chrysoberyl had lost its cat’s eye look. After asking her for more detail, she informed me that a few days prior it had been cleaned by a friend. What transpired was that the friend had not only cleaned the top of the cabochon cut gem, but also polished the flat surface underneath. The result was that light which was previously locked in the gem, now seeped through the bottom.