Two geological formations are important in connection with “Fairburns”: the Minnelusa formation - in those the agates developed - and the Chadron formation, which consist of eroded remainders of the Minnelusa formation. The Black Hills are offering several agate-hunting places of the over 300 million years old Minnelusa formation.

The legendary agates of the Teepee Canyon were first found in connection with road-construction-workings. In particular it is to be owed to the agate-collector Tom Wodden that many of these agates were saved. Agates from the TeePee Canyon are formed in flint - concretions, which can have a size up to a half meter. The agates from the Teepee - Canyon do have an sedimentary occurence. They are embedded in large flint-concretions.Those concretions can reach sizes up to one and a half feet. The agates in the middle are much smaller then the outer flint-krust.

Common are yellowish and orange colours. But red and purple colours do occur too.