I found this (5/8") together with a bunch of old agates, German swirls and clays. I think it is polished natural yellow limestone with manganese banding (Gartley and Carskadden, p. 105 & Fig. 53). According to the authors, it appears the stone "was being used for marble making in the second and third decades of the 19th century, and perhaps even later." They also claim, " . . . not many of these marbles are found on archaeological sites or in private collections". Does anyone have a feel for how common they are? Does anyone else have examples to share? Thanks for any input!