Chalcedony is a cryptocrystalline form of silica, composed of very fine intergrowths of the minerals quartz and moganiteHeaney, 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, whilst moganite is monoclinic.

Chalcedony has a waxy lustre, 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.

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:

Agate

Carnelian

Carnelian (also spelled cornelian) is a clear-to-translucent reddish-brown variety of chalcedony. Its hue may vary from a pale orange, to an intense almost-black coloration. Similar to carnelian is sard, which is brown rather than red.

Chrysoprase

Heliotrope

Moss agate

Moss agate (also known as tree agate or mocha stone) contains green filament-like inclusions, giving it the superficial appearance of moss or blue cheese. It is not a true form of agate, as it lacks agate's defining feature of concentric banding.

History

Structure

Chalcedony was once regarded to be a fibrous variety of cryptocrystalline quartz . More recently however, it has been shown to also contain a monoclinic polymorph of quartz, known as moganite. The fraction, by mass, of moganite within a typical chalcedony sample may vary from less than 5% to over 20% Heaney, Peter J., and Jeffrey E. Post. "The Widespread Distribution of a Novel Silica Polymorph in Microcrystalline Quartz Varieties." Science ns 255 (1992): 441-443. JSTOR. Aug. 2007. Keyword: moganite.. The existence of moganite was once regarded as dubious, but it is now officially recognised by the International Mineralogical AssociationOriglieri, Marcus. "Moganite: a New Mineral -- Not!" Lithosphere (1994). Aug. 2007 <http://fgms.home.att.net/moganite.htm>. Nickel, Ernest H., and Monte C. Nichols. "IMA/CNMNC List of Mineral Names." Materials Data. June 2007. Aug. 2007 <http://www.geo.vu.nl/users/ima-cnmmn/MINERALlist.pdf>..