Chatoyancy

Here’s a fancy French word, and if you know that chat is French for “cat,” you may have a clue what chatoyancy means: “shining like a cat’s eye.” The word refers to a quality of certain gemstones, not only cat’s-eye but moonstone and many others.

Something that is chatoyant possesses a changeable luster and has a narrow band of white light. If you change the position of the stone, the band of light seems to move across the surface.