A generally accepted requirement in the definition of Agate is that it has to be banded. This qualification distinguishes it from other forms of Chalcedony which are not banded. Exceptions include Dendritic Agate and Moss Agate, which are not true Agates since they lack the banding patterns, but they are still traditionally called Agates since they have more than one color. Onyx, when banded white and black, is technically a form of Agate, and Sardonyx, which is a banded reddish and white Chalcedony, is also technically a type of Agate. Some forms of Carnelian may also exhibit banding and can therefore be classified as both Carnelian and Agate.

Deposits where Agates are commercially mined are usually very extensive, thereby enabling this gemstone to be affordable and inexpensive. However, a fine and sharp banded pattern, combined with natural strong coloring, will quickly increase the cost and value of and Agate. Specific Agate localities will provide similarities in banding style and color, thereby lending many Agates a geographic prefix. Other variety names used will connote specific colors or patterns, such as Fire Agate or Eye Agate.

The history of Agate production is closely tied to German town of Ida-Bernstein, which has evolved as an important gemstone center. Agates and Jasper were historically found in the Ida-Bernstein region, and cut and polished by local craftsman. With the discovery of the enormous Agate deposits in Rio Grande do Sul, Brazil in the 1800’s, the new material was shipped to Ida-Oberstein for cutting and polishing. The local Agate cutters of Ida-Oberstein were well-suited for this job, having been historically trained in this profession. Ida-Oberstain remains today as an important cutting and polishing center.

In general, agate values are quite modest. Their prices reflect mainly labor and artistry rather than the value of the material itself. Agates of large size or with particularly distinctive, fine, or landscape-like color patterns are at a premium. Custom cut pieces or stones from collectible locations would be substantially more expensive.