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 the places of discovery are Pringle, the Hell Canyon, the Custer State park and the Teepee Canyon. Besides there is a multiplicity of places of discovery, which are on private area, and are here published as anonymous locations in the black hills area.