What is chatoyancy in gemstones?

Chatoyancy is a captivating optical phenomenon observed in certain gemstones that is characterized by a band of light moving across the surface of the stone, creating the appearance of a "cat's eye." This effect occurs due to the presence of needle-like inclusions, often made of minerals such as rutile or asbestos, which align in a specific direction within the gemstone. When light hits the surface, these inclusions reflect and refract the light, resulting in the distinctive band of light that moves as the stone or the light source is shifted.

This feature is particularly prominent in certain gemstones like chrysoberyl, where the chatoyant effect can be quite striking, making such stones highly sought after for their unique visual appeal. The alignment of the needle-like inclusions is crucial; if they are not oriented correctly, the chatoyancy effect may not be visible. Thus, chatoyancy directly refers to the visual impact created by these internal needle-like inclusions rather than other optical properties or categories of gemstones.