An optical phenomenon, possessed by certain minerals in reflected light, in which a movable wavy or silky sheen is concentrated in a narrow band of light that changes its position as a mineral is turned. It results from the reflection of light from minute, parallel fibers, cavities or tubes, or needlelike inclusions within the mineral. The effect may be seen on a cabochon-cut gemstone, either distinct and well defined (such as the narrow, light-colored streak in a fine chrysoberyl cat's-eye) or less distinct (such as in the usual tourmaline or beryl cat's-eye).