Chatoyancy

Chatoyancy is an optical phenomenon in which a bright, narrow band of reflected light appears to move across the surface of a gemstone as the stone or light source is rotated. The term, derived from the French word chatoyer, is commonly described as the cat's eye effect. It is most famously seen in chrysoberyl but occurs in several other gemstones and materials.

It results from light interacting with aligned, fibrous inclusions or parallel cavities within the mineral, which act like tiny prisms or reflectors. The light band, or “eye,” is typically centered or off-center along the length of the stone and travels as the viewing angle changes. The effect is strongest when there is a single dominant orientation of inclusions. The classic example is chrysoberyl cat's eye (cymophane). Other examples include tiger's eye and cat's eye quartz, where parallel fibrous structures formed by altered minerals or replaced fibers produce the sheen.

To maximize chatoyancy in jewelry, stones are cut en cabochon with a smooth, curved surface, which presents the eye along the longest dimension. Faceting often destroys the effect. The appearance can be influenced by color, transparency, and the quality of the internal structure. Chatoyancy is distinct from asterism, which is caused by multiple intersecting directions of light reflection forming a star pattern.