Semiprecious quartz gem displaying chatoyancy, a vertical luminescent band like that of a cat's eye. Bands 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 a fine golden lustre when it is polished. The best stones come from South Africa. See alsocat's-eye.