chalcedony

a mineral, a cryptocrystalline variety of quartz. Chalcedony contains various admixtures, including Fe³⁺, Al³⁺, and up to 1–1.5 percent water. When viewed under the microscope, it reveals a slender fibrous, frequently divergent, structure. The fibers of quartz microcrystals are sometimes spiraled, as in rainbow chalcedony, which exhibits an iridescence. Chalcedony forms incrustations, reniform, botryoidal, stalactitic, or sinter aggregates, veins, concretions, and pseudomorphs after organic remains and minerals. It also fills cavities.

Chalcedony is translucent to transparent and has a waxy luster when fractured. Occurring in many different colors, it has no cleavage and an uneven or hackly fracture. It has a hardness of 6.5–7 on Mohs’ scale and a density of 2,570–2,640 kg/m³.

Numerous varieties of chalcedony are identified according to various characteristics. Varieties recognized on the basis of color differences include chrysoprase (green), sard (red-brown), carne-lian (reddish yellow), sapphirine (bluish gray), plasma (an onion-green cloudy chalcedony), and cer-agate (light yellow). Varieties with a spotted coloration include heliotrope (green with red spots), myrickite (gray with red spots), and point agate, or point chalcedony (white or light gray flecked with red points).

Varieties of chalcedony distinguished according to structure include various agates, which have concentric bands, and onyx, or banded agate, which has parallel straight or curved bands. Uruguayan agates combine concentric and horizontal bands. Various types of agates are distinguished according to the design of the bands, for example, fortification agate, ruin agate, landscape agate, star agate, and cloud agate.

Varieties distinguished according to inclusions include moss agate, a translucent variety with inclusions of green chlorite or actinolite, mocha stone (dendritic agate), which has dendritic inclusions of manganese or iron oxide, and enhydros, a variety consisting of nodules and containing water-filled cavities.

Chalcedony is a widely occurring rock-forming mineral. It is formed from low-temperature hydrothermal solutions, as well as during the processes of diagenesis, epigénesis, and weathering. It precipitates out primarily from colloidal solutions, products of the recrystallization of silica gels. Chalcedony is often found in sedimentary rocks in the form of nodules, concretions, sheet deposits, and pseudomorphs after shells, corals, and the like. Chalcedony is the primary component of many silica rocks, petrified wood, and jasper. The chief industrial deposits of chalcedony are associated with amygdaloidal effusive rocks and products of their disintegration.

Because of their diversity of color and ability to take a good polish, chalcedony and its varieties have been used since ancient times as gemstones and for making ritual objects and ornaments. Neolithic chalcedony and carnelian beads have been found, as well as chalcedony and carnelian cylindrical seals (Babylonia) and agate and carnelian insets in ancient Egyptian gold ornaments. Onyx was widely used in the ancient states of Middle Asia and the Mediterranean region.

Agate and chalcedony are also used in laboratory work for chemical and pharmaceutical purposes (agate mortars and pestles) and in precision instrument-making (reference prisms of weights and the like). In the USSR, deposits of gem-quality chalcedony and its colored varieties are found in Eastern Transbaikalia (Tuldun), the Urals (Magnitogorsk), Transcaucasia (Akhaltsikhe), and the Crimea (Karadag).