Chatoyancy is an optical effect that looks like a bright, moving band of light (a “cat’s-eye”) across a curved surface. It forms when aligned fibers, needles, or fine tubes inside a mineral reflect light into a concentrated line. Collectors see chatoyancy in materials like tiger’s-eye and in some tourmalines and feldspars. The effect is strongest on a polished cabochon cut and oriented correctly to the internal structure.
To evaluate Chatoyancy, use a single point light source and rotate the stone so you can see whether the band stays sharp and continuous. Orientation matters: the light band should be perpendicular to the direction of the internal fibers or needles that cause the effect. Rough pieces may show only hints; a proper cabochon reveals the full band. Record the band direction and strength because that guides cutting, display, and comparison between similar materials.
Chatoyancy vs. a glare spot Chatoyancy forms a moving band, not a single reflection point.
Chatoyancy vs. asterism Asterism shows multiple rays in a star. Chatoyancy shows one main band.
Chatoyancy vs. poor orientation Misaligned cuts weaken the band. The band should relate consistently to the internal fiber/needle direction.
Chatoyancy vs. surface scratches Scratches scatter light and blur the band. Evaluate on a clean, polished surface.