Figure 12. Photograph of small coal ball specimen from the Carbondale Formation, Illinois basin. Coal balls are concretions that occur within coal beds and are composed largely of carbonate minerals and occasionally of pyrite. Modified from a photograph by James Humphrey (Curragh Queensland Mining Ltd., Australia) in Atlas of Coal Geology, American Association of Petroleum Geologists (AAPG) Studies in Geology No. 45 (Papp and others, 1998); AAPG copyright 1998, reprinted by permission of the AAPG, whose permission is required for further use.