What is cat’s eye stone?

Nowadays cat’s eye gemstone is just the common referring term for the phenomenon of chatoyancy where moving the gem under a source of light gives the appearance similar to opening and closing of a cat eye. The optical phenomenon of the cat’s eye is a kind of iridescence, displayed on the surface of the stone in the form of a narrow floating light strip, which is formed as a result of the reflection of incident light from regularly located internal inclusions (parallel columns, tubes, plates). Initially the term "cat's eye" was applied to yellow-green chrysoberyl with the corresponding optical effect but later this name with the refinement of the mineral form where the effect was shown was applied to all other stones - tourmaline cat's eye, quartz cat's eye, heliodorous cat's eye and others.