onyx

onyx (ŏnˈĭks) [key], variety of cryptocrystalline quartz, differing from agate only in that the bands of which it is composed are parallel and regular. Its appearance is most striking when the bands are of sharply contrasting colors; black and white specimens are often used for cameos. Onyx was used in Roman times for the fabrications of vases and cups. Sardonyx contains onyx and carnelian or sard. "Onyx marble,""Mexican onyx," and "Oriental alabaster" are terms applied to travertine.

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