What Exactly is Chatoyance or Chatoyancy?
A rock or mineral that's chatoyant or exhibits chatoyancy (or chatoyance) dislays an optical phenomenon that's caused either by its fibrous structure or by fibrous inclusions or cavities that reflect light from within the stone. The effect can make colours and patterns look as if they're moving and is caused by a narrow band of reflected light beneath the surface of the stone.
To see chatoyancy in a rock or mineral it has to be gently tilted and moved around so that it reflects light from different angles. The optical phenomenon varies significantly from one stone to the next and is best seen in gemstones that have been polished as a cabochon.
The minerals best known for being chatoyant are tigers eye and chrysoberyl also known as cats eye. Chatoyancy can however be seen in several other minerals.
The word chatoyant originates from the French word for cat's eye.