Chatoyancy, is an optical reflectance effect seen in certain gemstones. Coined from the French word “œil de chat,” meaning “cat’s eye,” chatoyancy arises either from the fibrous structure of a material or from fibrous inclusions or cavities within the stone.
A lot of people have mistaken that cat’s eye is a type of gemstone with a band of concentrated yellow or whitish light across the stone, like the slit of the eyes of a cat. Now it must be stated clearly that the cat’s eye effect is a light phenomena and it occurs in a variety of gemstones.
When light strikes the surface of a gemstone some light will pass through it while some will be reflected back to the eye. When reflection occurs on the surface of a gemstone the amount and quality of light is called luster.
However, there may be a variety of gemstones where light reflection occurs slightly below the surface. This reflection is called sheen and the most striking example are those gemstones which produce the cat’s eye effect.
Normally cat’s eye gemstones have dense fine needle-like inclusions or tiny hollow tubes running parallel to each other and parallel to the plane of the base of the gemstone. Most gemstones which exhibit the chatoyancy effect are cut into cabochons to produce this phenomena, where the sheen is concentrated across the dome of the stone into an eye.
A good everyday example would be the light reflected from a spool of thread. Using a strong fiber optic light I beamed it at a spool of fishing thread and took a picture. Here, the cat’s eye effect is produced as shown in the picture.
The best known cat’s eye gemstone is cat’s-eye chrysoberyl, where an excellent stone can be more costly than a well cut diamond of similar weight. Top grade cat’s-eye is semi-transparent with a slightly greenish yellow body color and should be free from eye visible inclusions. The ‘eye’ should be silvery or yellow in color. When the ‘eye’ moves according to the light source, it should be narrow, bright, sharp and concentrated without any breaks or waviness.
Among the gemstones that exhibit the cat’s-eye effect are Alexandrite, Moonstone, Quartz, Tourmaline and Scapolite. Most of these gemstones must be cut into cabochons to exhibit the cat’s-eye effect. However, Tiger’s-eye Quartz is one of the few gemstones that shows strong chatoyancy even when cut into facets or flat surfaces.
Photo credit: gia.edu, amazon.in, geology.com, geogallery.si.edu, gemagsi.com