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 the over 300 million old Minnelusa formation in different places to get there for agate-hunting.

Also in the Spearfish-Canyon agate-hunting places are situated, in which the possibility consists to collect agates in situ.