Tiger-Eye and Alexandrite
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.
The stones distinguished as chatoyant sometimes include alexandrite, a variety of chrysoberyl, strongly dichroic and sometimes trichroic. Mr. Edwin W. Streeter, in his book Precious Stones and Gems, states that he has seen specimens of alexandrite with a perfect cat's-eye line, yet subject to the change of colour by artificial light characteristic of this mineral. To display the ray, the stone is of course cut convex instead of with six facets. This stone was discovered in the Ural Mountains, Siberia, in the year 1830, on an anniversary of the birthday of the Czar Alexander II., of Russia, for whom it was named. Alexandrite has marked hues of red and green, the national colours of Russia; by daylight it shows a bright or deep olive-green colour, but in artificial light a soft columbine red or raspberry red or raspberry tint. One description of this gem includes the phrase "it is an emerald by day and an amethyst by night." Subsequent to the discovery of alexandrite in the Urals, the same gem mineral, but of a better and more workable quality, was discovered in the island of Ceylon, which is the present principal source of supply.