Chatoyancy

An optical effect, possessed by certain translucent gemstones in reflecting light, in which a movable, wavy or silky sheen is concentrated in a narrow band of white light that changes its position, when the gem is turned. This is a characterized of cat's eye and some other minerals. It is caused by the reflection of light from numerous, minute, parallel fibers, cavities or elongated tubes, or needle-like inclusions within the mineral, oriented in accordance with the symmetry of the crystal. The effect may be seen in cut cabochon gems. It can be seen in certain quartz, chrysoberyl, tourmaline, moonstone, andalusite, scapolite, fibrolite, and adularia. When the needle-like or elongated cavities are parallel to more than one crystal face after being cutting cabochon, in other words, perpendicular to the plane of included materials, the stone exhibits a star effect. Also spelled chatoyance.