About the Sources and Properties of Cat's-Eye

CAT'S-EYE is a well established term in the trade in precious stones, and more than one mineral which exhibits chatoyancy--a French word signifying a changeable, undulating lustre, like the eye of a cat in the dark--is termed, and sold as "cat's-eye."

The true cat's-eye is cymophane, a variety of chrysoberyl, a mineral resembling beryl in containing the element glucinum (beryllium), but otherwise distinct. Chrysoberyl is devoid of silica, which beryl possesses, and is, theoretically, composed of glucina, 19.8 and alumina, 80.2. Jewellers variously call chrysoberyl "cat's-eye," "Oriental cat's-eye," or "Ceylonese cat's-eye." Besides its principal components, chrysoberyl frequently contains impurities such as iron and chromium oxides. Chrysoberyl is very hard--8.5, being third in Mohs's scale to the diamond, and when cut is susceptible of a high polish. Heavier than the diamond, the specific gravity of chrysoberyl ranges from 3.5 to 3.8. Chrysoberyl crystallises in the rhombic system and commonly appears in complicated twin crystals. This peculiar mineral has no distinct cleavage, but has a conchoidal fracture; it is brittle; acids will not attack it; it is infusible before the blowpipe; it can be electrified positively, by friction, and will remain charged for several hours. Lustre vitreous to slightly greasy. Chrysoberyl is transparent to opaque, but is only transparent when cut and polished; it is doubly, but not strongly, refractive. The limited range of colour in Brazilian specimens is from pale yellowish-green to golden yellow and brownish-yellow. Crystals from the Ural Mountains vary from an intense green to grass-green or emerald-green--the latter variety is alexandrite.

The distinction of cymophane from ordinary chrysoberyl is its chatoyancy, which appears as a milky-white, bluish or greenish-white, or, more rarely, golden-yellow sheen which follows every movement of the stone; this characteristic is most strongly developed by cutting the stone convex, and therefore cat's-eye is cut en cabochon. A silvery line or streak of light extends across the curved surface and is most strongly defined in a strong light, while its boundaries are sharpest in small stones. The effect of the chatoyancy is in great part due to the judicious work of the lapidary, and usually the greatest possible effect is produced by the greatest curvature of the surface. Chatoyancy appears only in the cloudy chrysoberyl, and the cloudiness is due to thousands of microscopically small cavities within the stone. The influence of the whims and preferences of royalty on the popularity of gems was remarkably illustrated by the sudden favour with which chrysoberyl cat's-eye was invested, when His Royal Highness, the Duke of Connaught, gave his fiancee a ring set with this stone, which vastly increased the demand for it and caused a corresponding rise in price.

The Minas Novas district in the northern part of the state of Minas Geraes, Brazil, is the most prolific producer of chrysoberyl of the finest colours; most of the specimens are chatoyant. The mineral in this locality occurs associated with rock crystal, amethyst, red quartz, green tourmaline, yellowish-red (vinegar) spinel, garnet, euclase and white and blue topaz. Chrysoberyl is erroneously indentified with, and termed, chrysolite by the Brazilians, and this error is prevalent in the trade in precious stones and jewelry, almost everywhere. The usual tests, the scale of hardness especially, will promptly differentiate chrysolite. The source of supply of cymophane and non-chatoyant chrysoberyl second in importance to Brazil, is the island of Ceylon. The cat's-eye record for size was long held by a Ceylonese specimen, and, until the year 1815, this was a jewel in the crown of the King of Kandy. The weight of the Ceylon stones ranges from one to one hundred carats; they are found in company with sapphires in gem-gravels, chiefly in the Suffragan district and the vicinity of Matura in the south of the island. To a small extent, chatoyant chrysoberyl is mined in the Ural Mountains of Siberia.

Among the numerous minerals which when fibrous, or cut across the cleavage and convex, will exhibit the opalescent ray resembling the contracted pupil of the eye of a cat, are beryl, corundum, crocidolite, dumortierite, quartz, filled with pecicular crystals or fibrous minerals, such as actinolite, byssolite, or hornblende; hypersthene, enstatite, bronzite, aragonite, gypsum, labradorite, limonite, and hematite. These may be opaque, translucent, or transparent and of any colour or colours. Perhaps the commonest of these minerals is the quartz cat's-eye, which falls far short of rivalling the brilliancy and soft colouring of cymophane. The shades of this variety of quartz are greenish, yellowish-grey, and brown. Simple tests will distinguish this mineral from cymophane, as its hardness is but 6 to 7 and its specific gravity, 2.6. This quartz melts with soda to a clear glass, is soluble in hydrofluoric acid, and is not dichroic; its chief components are silicon and oxygen. Cut en cabochon, a band of light appears across the parallel fibres of asbestos which the quartz contains.

Tiger-eye, in the trade, is considered separately from cat's-eye, but as chatoyancy is its chief characteristic, it may as well be included here and, as its present commercial value is low and the demand for it is small, it can be summarily described and dismissed. The proper term for the mineral known as "tiger-eye" is crocidolite, a name derived from the Greek and meaning "woof," in allusion to its fibrous structure. Crocidolite is a fibrous asbestos-like mineral. Its colours are gold-yellow, ranging to yellowish-brown, indigo to greenish-blue, leek-green and a dull red. The blue is usually distinguished as "hawk's-eye." Crocidolite contains a siliceous base, usually a ferruginous quartz, and when cut highly convex with the longer diameter of the oval at right angles to the direction of the fibres, the cat's-eye ray is strongly apparent. Crocidolite contains: silica, 51; iron oxides, 34; soda, 7; magnesia, 2; water, 3. Hardness 4 to 7 and specific gravity 3.26. The best specimens are found in the Orange River region and Griqualand, South Africa.

Tiger-eye is well adapted to, and has been largely used for carving cameos and intaglios; it was very popular from about the year 1880 to 1890 in the United States.