South Dakota

The State's best known chalcedony is its colorful and beautiful Fairburn agates. Named after a community near a very prolific agate deposit in Custer County, these brightly colored banded agates are similar to Lake Superior agates found in Michigan and Dryhead agates from Montana. The color patterns are alternating bands with one of the bands always white. The colors that alternate with white include yellowish-brown, dark red, salmon pink, black, yellow, grayish-blue, and milky-pink.

The agate nodules range in size from about 20 millimeters in diameter to some that weigh as much as 20 kilograms. The nodules are recovered from the weathering of the Chadron formation in an elongated belt covering parts of Custer, Pennington, and Shannon Counties, with the community of Fairburn at about the center of the belt. Nodules similar to the Fairburn nodules weather out of a limestone formation in an area that includes parts of Custer and Fall River Counties.

Other varieties of agate are found in the State. Moss agate, much like the famous Montana moss agate, can be found in river gravels of the Little Missouri River system in Harding County. A wide variety of agate can be found in the gravel pits in the entire eastern part of the State.