chalcedony
chalcedony
A fine-grained or cryptocrystalline variety of quartz; commonly microscopically fibrous; translucent or semitransparent, with a nearly waxlike luster; has lower density and indices of refraction than ordinary quartz. Chalcedony is the material of much chert, flint, and jasper; commonly an aqueous deposit filling or lining cavities in rocks. In the gem trade, the name refers specif. to the light blue-gray or common variety of chalcedony. Varieties include carnelian, sard, chrysoprase, prase, plasma, bloodstone, onyx, and sardonyx. Syn:calcedony, chalcedonite; white agate.
A general name for crystalline silica that forms concretionary masses with radial-fibrous and concentric structure and that is optically negative (unlike true quartz).
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