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Among the gemstones that display special optical phenomena, cats eye gems are particularly interesting, in part because the effect is found in so many different gem varieties. The technical term for the cats 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 parallel to the fibers for this effect to be displayed. When the gem is rotated, the cats eye appears to glide over the surface. The chatoyant effect is similar to asterism (the star effect), except there is one straight ray instead of four or six. Occasionally a cats eye with two parallel rays can be seen.
Natural Chrysoberyl cats Eye
The most famous and valuable cats eye gemstone is chrysoberyl cats eye. In fact when the term cats eye is used alone in the gem trade, it always refers to chrysoberyl cats eye. All other types of cats eye gems require an additional varietal designation, such as cats eye apatite.
Chrysoberyl cats 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 cats eye is found in Sri Lanka, Brazil, India, China and Zimbabwe.
The cats 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, cats 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, cats 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.
Natural cats Eye Apatite
Among the softer gemstones, the cats 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, cats eye apatite is most suitable for pendants, brooches and earrings.
The cats 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.