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 quartzes which are not macrocrystalline, such as amethyst, citrine, rock crystal, and smoky quartz are referred to generically as chalcedony.
Chalcedony is also used as a variety name to refer specifically to the bluish-gray or lavender color of cryptocrystalline quartz. This color is sometimes called "actual chalcedony" or "chalcedony in the narrow sense." This can be slightly confusing, but fortunately most of the other chalcedonies are referred to by other names, such as agate, bloodstone or carnelian.
Chalcedony was once thought to be a fibrous variety of cryptocrystalline quartz. More recently however, 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 fraction, by mass, of moganite within a typical chalcedony sample may vary from less than 5% to over 20%.
Though the various chalcedonies are quartz, they have a very different look from the macrocrystalline quartzes such as amethyst. Chalcedonies are typically opaque to translucent, with a waxy to greasy or dull luster; crystal quartz is usually transparent with a vitreous luster.
Chalcedonies are most often cut as cabochons and with their good hardness (7 on the Mohs scale) and absence of cleavage they are 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) and 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 chalcedonies are 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 fashioning stones for jewelry, carvings -- including cameos and intaglios utilizing banded varieties such as agate, onyx, and sardonyx.