Noun
A variety of onyx consisting of sard and white chalcedony in alternate layers.
Source: Webster's dictionaryAgate is a frequent material, especially carnelian or banded agate like sardonyx ; the banding makes the impression contrast with the ground. Source: Internet
Holland, pp. 294–295; Southern, p. 108 The Blacas Cameo showing Augustus wearing a gorgoneion on a three layered sardonyx cameo, AD 20–50 Soon after his bout of illness subsided, Augustus gave up his annual consulship. Source: Internet