Is that maybe cement that was left in the bottom of a 5 gallon bucket?
If it had some kind of axis and was smooth around the edges, I could imagine it being a wetstone wheel.
Thanks for the welcomes! It does not have any definite cupped area, but some intresting wear in a couple places (could be from the creek though.) I will have to try to put in a bucket tonight when I get home, kinda hope it doesn't fit!!!