Keokuk geodes attract rock-hunting hobbyists from all over, but just how easy is it to find one? “As a young kid, we used to go to the Mississippi over there and go fishing,” Mike Shumate recalled.

Göttingen University researcher Fiorella Arduin Rode working in open pit mine of the study area, Los Catalanes mining district, northern Uruguay. Along with Ametista do Sul in southern Brazil, these ...