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A general term for several varieties of gemstones that, when suitably cut en cabochon and viewed in a certain direction and light, show chatoyancy resembling a moving streak of light as seen in a cat's eye. The streak is caused by reflection from parallel layers of asbestos fibre or from minute internal canals.
The stones, which have a silky lustre, include: a variety of chrysoberyl (known as cymophane); a variety of greyish-yellow, brownish or grey-green quartz, similarly chatoyant but inferior, having a less sharp streak and lower hardness, and which, when replacing crocidolite in a stone, is known as tiger's-eye and hawk's-eye; a variety of tourmaline that has fibrous inclusions; some varieties of diopside, moonstone, andalusite, apatite, scapolite, and fibrolite (sillimanite); and some of the varieties of corundum.
All such stones are preferably designated as 'cat's-eye' accompanied by the name of the stone; the use of the term alone, without the name of a stone, should be restricted to designate a chrysoberyl. The stones must be cut with meticulous care so that the streak will cross the middle of the dome when light falls upon it vertically. The stones are used for beads or set in finger rings.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson