What Exactly is Chatoyance?
Chatoyance is an optical reflectance which can be seen in certain stones the best known of which is probably the mineral Tigers Eye. Originating from the French word for cat's eye, chatoyance is caused because of the fibrous structure of a mineral, the fibrous inclusions or cavities which reflect light from within the stone. It can usually be seen as the stone is gently moved back and forth hence enabling it to catch light at different angles, the reflections give the impression that visible patterns and markings are moving.
The effect of chatoyance may be likened to the reflection of light on a spool of silk thread, as the light reflects off the thread it forms a line of light in one specific section.