Chalcedony ( kal-SED-ə-nee, or KAL-sə-doh-nee) 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) and is probably derived from the town of Chalcedon in Turkey. The name appears in Pliny the Elder's Naturalis Historia as a term for a translucent kind of jaspis. Another reference to a gem by the name of khalkedón (χαλκηδών) is found in the Book of Revelation (21:19); however, it is a hapax legomenon found nowhere else in the Bible, so it is hard to tell whether the precious gem mentioned in Revelation is the same as the mineral known by this name today. The term plasma is sometimes used to refer to green translucent chalcedony.