I live in south central Michigan- sandstone, found in my garden while digging!! Anybody know for sure? If you look closely you can see a few divots that run perpendicular to the larger one. Backside is curved to fit perfectly in the hand.
Here are some more I found today. same location, a few inches deeper. the two pieces of rock that fit together were found separate, only after bringing them in and washing them off did i see that they fit together.
Could we see some more pictures of the first piece? It does look like an Abrader and there would have been another side so you could put the two pieces together and rub along the shaft.
Here are some I have from a Mandan site near Bismark ND.