Cat's Eye Tourmaline
The optical phenomenon is exceptionally rare and occurs in only a handful of gems. The distinct reflection is caused by tiny parallel fibers or needle-like inclusions within the stone, which is best viewed under a direct light source, ideally sunlight.
Cat's eye tourmaline has superior durability and in most cases, it can be easily identified by its distinct hardness. For example, it is harder than both quartz and apatite, but softer than chrysoberyl, so distinguishing it from these is rather easy.
Cat's eye tourmaline is not known to be heated, treated or enhanced in any way, rendering it one of the few untreated gems available today.