Agate is a common, semiprecious type of chalcedony. Most agates form in cavities in ancient lavas or other extrusive igneous rocks. The characteristic bands in the agate usually follow the pattern of the cavity in which the mineral has formed (curved, often semi-concentric bands). The colors of those bands are determined by the different impurities present, materializing in different colors like white, yellow, gray, pale blue, brown, pin, red, or black. The variety of sliced agate in the market in particular vivid bright colors is commonly dyed or stained to enhance the natural color. Dyeing agate or stain enhancing it is very easy due to its porous nature.
The name agate is believed to have derived from the river Achates (now called the Drillo) in Sicily. The most important deposits of agate today are in the south of Brazil (Rio Grande do Sul) and in the north of Uruguay. Other deposits are in Australia (Queensland), China, India, the Caucasus, Madagascar, Mexico, Mongolia, Namibia, and the United States (Wyoming and Montana).
The South American deposits produce agates that normally appear opaque gray and without markings. Only when dyed do they obtain their coloring and lively patterns. The art of dyeing was known to the Romans. In Idar-Oberstein/Rhineland-Palatine, a city in the Rhineland of southwest Germany, agate dyeing has been practiced since the 1820s; the perfection of their dyeing has not been achieved anywhere else in the world. Therefore, it has developed into the most important center in the world, for the cutting of agate and other stones.
Depending on the sample, design, or structure of the agate layer there are different names of agate: Fortification agate, (type of banded agate with angular arranged bands); Botswana agate from Africa has beautiful dark and light banding; Mexican lace agate, (sometimes called “crazy lace”) is a multicolored fortification agate; Blue lace agate from South Africa (one of the most common kinds for sale today), is a delicate light blue with a fine inter-layering of colorless agate; Fire agate (inclusions of red to brown hematite that give an internal iridescence to polish stones; Banded agate (produced by a series of processes that take place in cavities in a solidified lava); Moss agate (translucent chalcedony with moss-like inclusions of hornblende or chlorite; Sweetwater agate from the Sweetwater River area of Wyoming is characterized by fine, black dendrites; Eye agate (ring-shaped design with point in the center, similar to an eye; Layer agate (layers and bands of about the same size parallel to the outer wall of the agate nodule); Scenic agate (shows landscape-like images through dendrites); Pseudo-agate (also called polyhedric quartz), its interior similar to agate with layering and druse opening of a geometric shape; Tubular agate (agate with numerous tubes); thunder egg or sandstone, a layered agate nodule with strongly furrowed outer surface; and Brecciated agate, which is an agate broken, but cemented together by quartz.
Ancient Egyptians used agate as gemstones almost 3,000 years ago. Today it is used in the arts, for decorative purposes, beads, rings, brooches, pendants, and as layer stones for cameos.