"Chatoyant" is derived from the French "oeil de chat" which means cat's eye. Cat’s eye is a special optical phenomenon that is found in many different gemstone varieties. It arises either from the fibrous structure of a material, as in tiger's eye quartz, or from fibrous inclusions or cavities within the stone as in cat's eye chrysoberyl. It is not a gemstone per se.

The gemstone needs to be cut and polished into a cabochon in such a way that it displays a narrow band of concentrated light going across the width of the stone. This chatoyancy is caused by inclusions of fine, slender parallel mineral fibers in the gemstone that reflect light in a single band. This effect looks similar to the slit eye of a cat.

The cat’s eye seems to appear to glide over the surface of the cabochon when the gemstone is turned over a beam of light.

Asterism is not chatoyancy. In the star effect (or asterism) there are four or even six rays that form a star on the surface of the gemstone.
Sometimes a chatoyant or Cat’s Eye gemstone will have two parallel rays.

Chrysoberyl cat’s eye is the most well-known and also the most highly priced as the chatoyancy in chrysoberyl is very prominent. In the gemstone market, when the word cat’s eye is used, it always refers to Chrysoberyl Cat’s Eye.

Other gemstones are given a cat’s eye label preceded by the gemstone’s names. For example, cat's eye Tourmaline, cat's eye Apatite, cat's eye Diaspore, cat's eye Moonstone and so forth. There is also Tiger’s Eye.

Chrysoberyl and Tiger's Eye are two of the best-known gemstones that display chatoyancy. Chrysoberyl displays the finest cat’s eye effect, whereas, Tiger’s Eye is the most often used chatoyant gemstone in jewelry.

Other gemstones that exhibit the cat’s eye effect are actinolite, beryllonite, cerussite, danburite, diopside, enstatite, garnet, kyanite, opal, peridot, pezzottaite, prehnite, quartz, rutile, sillimanite, spinel, topaz, tourmaline and zircon.

The cat's eye effect can also be seen in a number of other gem varieties, but this chatoyancy is extremely rare. They include emerald, iolite (also known as cordierite), aquamarine, andalusite, tanzanite and scapolite. These are real collector's items.

Chrysoberyl cat’s eye has a hardness of 8.5 on the Mohs scale, therefore making it perfect for all types of jewelry.
Note: An alexandrite cat's eye is a chrysoberyl that changes color.

The chrysoberyl family consists of three of the world’s rarest gemstones: the transparent color-changing Alexandrite, the Chrysoberyl Cat’s Eye, and the high quality transparent Chrysoberyl gemstones that come in yellow, green, brown and colorless. Chrysoberyl Cat's Eye deposits are found in Sri Lanka, Brazil, India, China and Zimbabwe.

Cat’s eye Tourmaline which is another high-quality gemstone with a cat’s eye effect, has a hardness ranging from 7 to 7.5 on the Mohs Scales which is hard enough to be made into rings. Cat’s eye Tourmaline can often be found in larger sizes in green and pink colors.

A quartz variety gemstone with cat’s eye effect is Tiger’s Eye which comes in gold-yellow and gold-brown colors. Tiger’s Eye was first formed as blue Crocidolite which is of an iron and sodium chemical composition. Crocidolite is often replaced by quartz, forming the chatoyant semiprecious gems tiger's eye and hawk's-eye.

Another gemstone that displays the cat’s eye effect is Apatite which comes in similar golden color to that of Chrysoberyl. Cat's eye Apatite can also be found in white (colorless), pink, yellow, green, blue, and violet colors.

Apatite has a hardness of 5 on the Mohs Scale. As it isn't very hard, it is only good for pendants, brooches and earrings, but not rings which are more susceptible to knocks and scratches.