Waco Gem and Mineral Club, Waco, Tx

17.4 Chatoyancy Cats Eye and Asterism

 

17.4: Chatoyancy: Cat‘s Eye and Asterism

Cat lovers should enjoy chatoyancy. It‘s from a French word, chatoyer, meaning to shine like a cat‘s eye. Chatoyancy is commonly called cat’s-eye effect. In bright light, a cat‘s pupil narrows to a vertical slit. When some gemstones are rounded and polished, bright light will be reflected as a single thin ray, looking very much like a cat‘s eye.

Chatoyancy is caused by inclusions, or minerals enclosed within another mineral. Light entering the host mineral reflects off included minerals. When inclusions are fibrous and run parallel to one another, they produce a single line of reflected light running perpendicular to the direction of the fibers. You can illustrate this effect for kids using a spool of sewing thread. Hold it under a light, and they‘ll see a vertical line running perpendicular to the wound thread. Chatoyancy is enhanced if the stone is rounded into a cabochon or sphere, concentrating the light, just as with our rounded spool of thread.

Chatoyancy can be produced by many minerals and inclusions. Yellow cymophane, or chrysoberyl containing rutile or tube-like cavities, is highly valued. A golden cat‘s-eye quartz contains rutile, while a gray-green variety contains amphibole asbestos. A gray-blue quartz containing partially silicified crocidolite (blue asbestos) is called hawk‘s eye. The popular tiger‘s eye quartz forms when crocidolite fibers are replaced by silica along with iron oxides, producing a silky golden color. Then there‘s actinolite, apatite, beryl, tourmaline, scapolite, moonstone, and more. You can even observe chatoyancy with common, non-precious minerals such as satin-spar gypsum or ulexite.

Minerals producing a chatoyant effect are found worldwide, but some places are especially famous: Sri Lanka and Brazil for cat‘s-eye chrysoberyl and quartz; South Africa and Australia for tiger‘s eye; California, for cat‘s-eye tourmaline. In addition to appreciating the beauty of cat‘s-eye gemstones, some cultures consider them good-luck charms with the ability to counteract an 'evil eye'.

The Greek word aster means 'star', and asterism refers to a luminous star-like figure appearing on the face of a gemstone as a result of reflected light. Asterism is similar to cat‘s eye. Like cat‘s eye, it‘s caused by included fibers that run parallel to one another, producing a single line of reflected light. If bundles of such fibers are oriented in two directions, they‘ll produce two intersecting eyes, resulting in a four-rayed star. Oriented in three directions at 120 degrees to each other, as may happen within hexagonal crystals of corundum or quartz, rutile bundles will create three eyes, or a six-rayed star.

Cabbing focuses and concentrates light to produce the star effect, but much can go wrong in the process. The gem cutter must orient the base of the stone parallel to the plane of the inclusions, or the star may be off-centered. Another decision: how high to dome the stone? A higher dome channels light more effectively, creating a sharper star. But if the dome is too steep, rays get cut off and don‘t wrap around the surface of the dome. A dome cut too flat produces a fuzzy, ill-defined star. Gemstones most associated with asterism are star rubies and sapphires. However, Idaho produces star garnets. Quartz, diopside, and spinel can also exhibit asterism.