Chalcedony is an umbrella term for a specific variety of quartz, or silicon dioxide, which comes in an assortment of sizes and colors. As a general rule, it is translucent to semi-transparent, and often has a milky or obscured color. It also takes well to polish, and some varieties seem to glow from within after prolonged polishing. The stone is typically classified by color, as chemically all chalcedony is identical. Popular examples include carnelian, agate, bloodstone, flint, and jasper, among many other stones.
The structure of chalcedony is classified as microcrystalline or cryptocrystalline, meaning that the crystalline structure of the rock is so fine that it is not readily visible to the naked eye. In order to identify the individual crystals which make up the stone, it must be sliced into thin sections and viewed under a microscope with a polarized light source. Even then, the crystals are usually quite difficult to distinguish. This makes the stone relatively durable and easy to work with, far less prone to fracturing that other forms of quartz or minerals with larger crystal structures.
Chalcedony typically forms over a long period of time, as minerals are deposited into a pocket in another form of rock such as granite. Areas of volcanic activity frequently harbor deposits of it. Typically, the silicon dioxide forms parallel bands, which are sometimes readily visible, as is the case with many agates. In other cases, the separate deposits may appear blended to the eye, as with carnelians, moss agates, and many other forms of chalcedony. The colors are caused by trace amounts of other impurities in the silicon dioxide. Pure chalcedony without any trace minerals, often called milky quartz, is also found in some regions. Some types are more rare or sought after than other, causing costs to vary widely depending on the variety and the current market.
Examples of chalcedony can be found all over the world, although some regions are better known than others for the production of high quality stones. It has been used for centuries in decorative art and jewelry, and often appears in the form of carved cameos, beads, or simple stone settings on rings, necklaces, and bracelets. Many societies associated particular traits with certain varieties of chalcedony, and many religious icons and symbols such as crosses, worry beads, and ornaments are made with various forms of it. It is also a popular choice of stone for many modern jewelers, who appreciate the range of colors and prices that it offers.