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chalcedony
Chalcedony is a cryptocrystalline form of silica, composed of very fine intergrowths of the minerals quartz and moganite.〔Heaney, Peter J., 1994. Structure and Chemistry of the low-pressure silica polymorphs. In: Reviews in Mineralogy v. 29; Silica: Physical Behavior, geochemistry and materials applications. Ed. Heaney, P.J., Prewitt, C.T., Gibbs, G.V., 1-40〕 These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic. Chalcedony's standard chemical structure (based on the chemical structure of quartz) is SiO2 (silicon dioxide).
Chalcedony has a waxy luster, and may be semitransparent or translucent. It can assume a wide range of colors, but those most commonly seen are white to gray, grayish-blue or a shade of brown ranging from pale to nearly black. The color of Chalcedony sold commercially is often enhanced by dying or heating.〔
The name ''chalcedony'' comes from the Latin ''chalcedonius'' (alternatively spelled ''calchedonius''). The name appears in Pliny the Elder's ''Naturalis Historia'' as a term for a translucid kind of Jaspis.〔Pliny the Elder: Naturalis Historiae, Book 37, chapter 7,115 ((online ))〕 The name is probably derived from the town Chalcedon in Asia Minor.〔Erika Zwierlein-Diehl: Antike Gemmen und ihr Nachleben. Berlin (Verlag Walter de Gruyter) 2007, S. 307 ((online )) According to the OED a connection with the town of Chalcedon is "very doubtful":〕 The Greek word ''khalkedon'' (χαλκηδών) also appears in the Book of Revelation (Apc 21,19). It is a hapax legomenon, a word found nowhere else, so it is impossible to tell whether the precious gem mentioned in the Bible is the same mineral known by this name today.
== Varieties ==
Chalcedony occurs in a wide range of varieties. Many semi-precious gemstones are in fact forms of chalcedony. The more notable varieties of chalcedony are as follows:
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