Chalcedony

Chalcedony, mineral, a cryptocrystalline variety of quartz of various shades of white, gray, yellow, brown, green, and blue. Like quartz it has the formula SiO₂, a hardness of 7, and a specific gravity of 2.65. Although it is usually translucent, some of the milk-white varieties are opaque. The color variations occur as the result of the presence of such impurities as iron, aluminum, and nickel. Chalcedony occurs as a lining or filling of cavities in rocks. The principal varieties are agate, which is banded and striped, having alternating layers of chalcedony and opal; jasper, or other forms of quartz; carnelian, which is clear and of various shades of red; chrysoprase, an apple green variety, in which the color is due to nickel oxide; heliotrope or bloodstone, of a dark green color, with small spots of jasper; onyx, consisting of bands of opal and chalcedony of different colors, usually black and white; plasma, of a deep green color; and sardonyx, a red-and-white-banded variety of chalcedony.

The many colors and the high luster that chalcedony takes by polishing render it valuable for brooches, necklaces, and other ornaments, and some varieties are cut as sealstones. Chalcedony is found in many parts of the world.