not sure rock but some got more abused than others,it could be something else but i call it a hammer stone.they would wrap these up in rawhide too and use as a war hammer,or maybe grind corn or other grains who really knows.they would make arrowheads with them too.i would think they would like to use a smooth rock rather than a rough one to work with as it is better on the hands.could of been on its way to be a celt maybe too.