semiprecious quartz gem displaying chatoyancy, a vertical luminescent band like that of a cat's eye. Veins of parallel, blue asbestos (crocidolite) fibres are first altered to iron oxides and then replaced by silica. The gem has a rich yellow to yellow-brown or brown colour and, when polished, a fine golden lustre. The best stones come from Griqualand West, S.Af. Hawk's-eye is similar to tigereye, except that the crocidolite was replaced by quartz before altering to iron oxide; it therefore retains the gray-blue or green of the asbestos. See also cat's-eye.