La calcedonia es un mineral de sílice (SiO2) de estructura microcristalina. Tradicionalmente considerada una variedad de cuarzo, más recientemente se ha visto que en realidad es una mezcla de cuarzo con moganita, mineral de igual fórmula química pero monoclínico, produciendo la mezcla los bandeados típicos de algunas de las variedades de calcedonia.

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Chalcedony (pronunciation: /kælˈsɛdəni/) is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. 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 dyeing 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. The name is probably derived from the town Chalcedon in Asia Minor. The Greek word khalkedon (χαλκηδών) also appears in the Book of Revelation (Apc 21,19). It is a hapax legomenon (the term for a word found nowhere else) so it is hard to tell whether the precious gem mentioned in the Bible is the same mineral known by this name today.

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