The Fairburn Agate is the state stone in South Dakota. It is named after the town from which the first agates were found: Fairburn, SD. This is an unusual agate, in that they formed in sedimentary rock, rather than igneous rock in which most other agates formed. This sedimentary genesis probably contributed the minerals which cause the brilliant color of Fairburns. These agates are also known for the extreme color contrast in the banding. The agate bands typically have sharp peaks, often resembling a maple leaf pattern. Most Fairburns have concentric fortification banding.
These agates are found in the grassland and badland areas surrounding the Black Hills, located in the southern part of the state. It is believed that the sedimentary rock matrix in which the agates formed were uplifted when the Black Hills formed. As the limestone eroded, the hard agates eroded out. As they moved downstream with eroded material, many of the Fairburns were rounded and river polished.
The value of a Fairburn agate is determined by size (most are small—only a few ounces), color, quality of banding, number of bands, shape of the fortification pattern, and condition (fracture free). As far as I know, a Fairburn agate still has the record for the highest priced agate sold on EBay. Several years ago, a Fairburn agate sold for $13,000.