1911 Encyclopædia Britannica/Cat’s-Eye

by Frederick William Rudler

CAT’S-EYE, a name given to several distinct minerals, their common characteristic being that when cut with a convex surface they display a luminous band, like that seen by reflection in the eye of a cat. (1) Precious cat’s-eye, oriental cat’s-eye or chrysoberyl cat’s-eye. This, the rarest of all, is a chatoyant variety of chrysoberyl (q.v.), showing in the finest stones a very sharply defined line of light. One of the grandest known specimens was in the Hope collection of precious stones, exhibited for many years at the Victoria and Albert Museum. (2) Quartz cat’s-eye. This is the common form of cat’s-eye, in which the effect is due to the inclusion of parallel fibres of asbestos. Like the chrysoberyl, it is obtained chiefly from Ceylon, but though coming from the East it is often called “occidental cat’s-eye”—a term intended simply to distinguish it from the finer or “oriental” stone. It is readily distinguished by its inferior density, its specific gravity being only 2.65, whilst that of oriental cat’s-eye is as high as 3.7. A greenish fibrous quartz, cut as cat’s-eye, occurs at Hof and some other localities irr Bavaria. (3) Crocidolite cat’s-eye, a beautiful golden brown mineral, with silky fibres, found in Griqualand West, and much used in recent years as an ornamental stone, sometimes under the name of “South African cat’s-eye.” It consists of fibrous quartz, coloured with oxide of iron, and results from the alteration of crocidolite (q.v.). It is often distinguished as “tiger’s-eye” (or more commonly “tiger-eye”), whilst a blue variety, less altered, is known as “hawk’s-eye.” By the action of hydrochloric acid the colour of tiger’s-eye may to a large extent be removed, and a greyish cat’s-eye obtained. (4) Corundum cat’s-eye. In some asteriated corundum (see Asteria) the star is imperfect and may be reduced to a luminous zone, producing an indistinct cat’s-eye effect. According to the colour of the corundum the stone is known as sapphire cat’s-eye, ruby cat’s-eye, topaz cat’s-eye, &c.  (F. W. R.*)