Cat's Eye Gemstones
Among the gemstones that display special optical phenomena, cat's eye gems are particularly interesting, in part because the effect is found in so many different gem varieties. The technical term for the cat's eye effect is chatoyancy, derived from the French for eye of the cat ("oeil de chat").
The chatoyant effect resembles, appropriately enough, the slit eye of a cat. The effect is caused by the reflection of light by parallel fibers, needles or channels in the gemstone. Usually the gemstone needs to be cut en cabochon with the base in parallel with the fibers for this effect to be displayed. When the gem is rotated, the cat's eye appears to glide over the surface. The chatoyant effect is similar to asterism or the star effect, except there is one straight ray instead of four or six. Occasionally one will find a cat's eye with two parallel rays.
The most famous and valuable cat's eye gemstone is chrysoberyl cat's eye. In fact when the term cat's eye is used alone in the gem trade, it always refers to chrysoberyl cat's eye. All other types of cat's eye gems require an additional varietal designation, such as cat's eye apatite.
Chrysoberyl cat's eye belongs to the same family as the rare alexandrite. Since chrysoberyl is a very hard stone (8.5 on the Mohs scale), it is suitable for all kinds of jewelry. Chrysoberyl cat's eye is found in Sri Lanka and Brazil, with additional deposits in India, China and Zimbabwe.
The cat's eye effect can be found in a number of other gem varieties, though in some cases the effect is quite rare. Among the finer gemstones, cat's eye tourmaline can often be found in green and pink, and larger sizes are not uncommon. With a hardness of 7 to 7.5 on the Mohs scale, cat's eye tourmaline is durable enough for rings.
Several quartz varieties are well-known for their chatoyant effects. The most famous is so-called tiger's eye. Found in gold-yellow and gold-brown, it is first formed as the fibrous blue mineral called crocidolite, which is made up of iron and sodium. The crocidolite was gradually transformed into quartz while maintaining its fibrous formations.
Among the softer gemstones, the cat's eye effect is often found in the gemstone apatite, in a golden color somewhat reminiscent of chrysoberyl. It can also be found in colorless, pink, yellow, green, blue and violet. But with a hardness of only 5 on the Mohs scale, cat's eye apatite is most suitable for pendants, brooches and earrings.
The cat's eye effect can also be seen in a number of other gem varieties, but only very rarely. They include emerald, iolite (also known as cordierite), aquamarine, andalusite, tanzanite and scapolite. These are real collector's items if you can find them.
- First Published: June-05-2008
- Last Updated: October-06-2010
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