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.

Among the most well-known ranks the fagate-finding zone from Hell Canyon. In the year 2000 by a forest fire wide ranges of the vegetation were destroyed. At these now “open” places could be found agates. Often they are intensively red colored and show partially a beautiful spherulitical pattern. The agates usually occur in flint-concretions, which formed in the limestone of the Minnelusa formation.