Pumices and ore of stones in a riverbed (base rock) assuming the spherical or disc shapes, were used unprocessed as materials, and many of these left the traces of being ground as signs of being used.
球状または円盤状を呈する転石や河原石の原石（母岩）を加工せずにそのまま用いており、使用痕としては擦った痕跡ののこるものが多い。