What is chatoyancy in gemmology ?

Chatoyancy : definition

Chatoyancy refers to a particular phenomenon of changing reflections within a crystal, which then gives it a silky appearance. It results from the presence of a multitude of fibrous micro-inclusions, fluid inclusions or cavities arranged in parallel planes. The reflection of light on these fibers or cavities perpendicular to their orientation gives this satin appearance, like that of a spool of thread.

This phenomenon results in the cat's eye effect. It is well illustrated by chrysoberyl, tiger's eye (oxidized crocidolite replaced by quartz), certain tourmalines, apatites, scapolites or certain feldspars, as well as many other mineral species.

The chatoyancy is also responsible for the asterism of sapphires, rubies, garnets, etc...
Tiger's eye from South Africa
Cat's eye opal from Tanzania
Cat's eye apatite from India
Star sapphire from Thailand