Chalcedony is the term used by gemologists as a species name for all types of cryptocrystalline quartz, including agate, chrysoprase, carnelian, bloodstone, jasper, onyx, moss agate and petrified wood. So all quartz that is not macrocrystalline, such as amethyst, citrine, rock crystal and smoky quartz, is referred to generically as chalcedony.
Chalcedony is also used as a varietal name to refer specifically to bluish-gray or lavender colored cryptocrystalline quartz. This color is sometimes called “actual chalcedony” or “chalcedony in the narrow sense”. This can be slightly confusing, but fortunately, most other chalcedony is referred to by other names, such as agate, bloodstone or carnelian.
Chalcedony was once thought to be a fibrous variety of cryptocrystalline quartz. However, more recently it has been shown to also contain a polymorph of quartz, known as moganite. While both quartz and moganite are silica minerals, quartz has a trigonal crystal structure, while moganite is monoclinic. The proportion, by mass, of moganite within a typical chalcedony sample may vary from less than 5% to over 20%.
Though the various types of chalcedony are quartz, they have a different look from macrocrystalline quartz such as amethyst, which is typically transparent to translucent and forms with larger crystals than cryptocrystalline quartz. Cryptocrystalline quartz forms with microscopically small crystals, which in most cases cannot be seen even under magnification.
Chalcedony is most often cut as cabochons and with its good hardness (7 on the Mohs scale) and absence of cleavage, it is suitable for all kinds of jewelry. A wide variety of colors are available; the most well-known include red-orange (carnelian), apple-green (chrysoprase), blue or lavender (chalcedony), black (onyx) and deep green with red (bloodstone or heliotrope).
The colors of chalcedony are not confined to solid hues. The deep green with red spots is known as bloodstone or heliotrope. Banded chalcedony is known as agate. Jasper is typically multicolored, spotted or flamed. Especially treasured pieces form patterns that look like natural landscapes.
Chalcedony was used to make cylindrical seals in Mesopotamia as early as the 7th century B.C. Subsequently, it has been used widely for jewelry and carvings, including cameos and intaglios that utilize banded varieties such as agate, onyx and sardonyx.