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, 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.