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Chalcedony is one of the cryptocrystalline varieties of the mineral quartz, having a waxy lustre. It may be semitransparent or translucent and is usually white to gray or grayish-blue or some shade of brown, sometimes nearly black. Other shades have been given different names. A clear red chalcedony is known as carnelian or sard; a green variety colored by nickel oxide is called chrysoprase. Prase is a dull green. Plasma is a bright to emerald-green chalcedony which is sometimes found with small spots of jasper resembling blood drops; it has been referred to as blood stone or heliotrope. The term chalcedony is derived from the Greek word meaning Chalkedon, a town in Asia Minor. Chalcedony is one of the few minerals other than quartz that is found in geodes.

See also: list of minerals