|Various Cat’s Eye Cabochons from Sri Lanka. Photograph: Jeffrey A. Scovil|
Cat's Eye describes a gemstone polished into a cabochon that displays a narrow band of concentrated light going across the width of the stone.
The name “chatoyance” originates from the French word “chatoyer,” which means “to shine like a cat’s eye.” The analogy also matches the way a cat’s pupils will narrow to a thin slit under bright light. Cat's eye effect is an optical reflectance effect seen in certain gemstones.
The precipitates that cause chatoyance in chrysoberyl are the mineral rutile, composed mostly of titanium dioxide. Examined samples have yielded no evidence of tubes or fibres. The rutile precipitates all align perpendicularly with respect to cat's eye effect. It is reasoned that the lattice parameter of the rutile matches only one of the three orthorhombic crystal axes of the chrysoberyl, resulting in preferred alignment along that direction.
Gem species known for this phenomenon include the aforementioned quartz, chrysoberyl, beryl (especially var. aquamarine), charoite, tourmaline, labradorite, selenite, feldspar, apatite, moonstone, thomsonite and scapolite amongst others. Glass optical cable can also display chatoyancy if properly cut, and has become a popular decorative material in a variety of vivid colors.
Famous examples of this type of sheen are Chrysoberyl Cat's-Eye (or "Cymophane") and Tiger's-Eye (Quartz with asbestos fibers).
|Unlike other cat’s-eye gemstones Chrysoberyl can exhibit a very sharp and distinct cat’s-eyes under almost any kind of light. (Photo by: Tino Hammid)|
Other gemstones that may show chatoyancy are:
|In order for the stone to exibit the cat’s eye effect it needs to be cut parallel to the fibers. If cut perpendicular to the fibers (90 degrees) you will end up with a dark lifeless gemstone. Photo: Frans Jonker G.G|
|Cat's Eye Minerals Collection. Photo: Frans Jonker G.|