Agate ("Fairburn Agate") from South Dakota, USA. (~3.15 centimeters across at its widest)

"Agate" is a rockhound/collector term for cavities in rocks (usually sedimentary rocks such as limestone or igneous rocks such as basalt) that have been partially or completely filled with irregularly concentric layers of microcrystalline, fibrous quartz (chalcedony - SiO2). Agate is quartz.

Attractive, multicolored and multipatterned agate has long been collected from a large area near the towns of Fairburn and Interior and south of the town of Kadoka and in the White River Badlands. This region has surficial, loose, late Cenozoic-aged gravels derived from weathering and erosion of bedrock in the Black Hills. Some of this gravel is agate. The Fairburn-area agates are remarkably colorful and desirable. The highest-quality examples have sold in the past for between 10,000 and 20,000 American dollars.