A rare Moroccan pseudomorph of a spiral
Turritella-like snail shell that has been replaced by chalcedony.
Chalcedony ( /kalˈsɛdəni/) 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).