Is that a type of sand stone or what. The bottom doesn't look real smooth and on most grinding stones they are very smooth from use. Also most are made out of a thicker stone JMO, but I would have packed it out just to have a good look at it.
I can't say for your neck of the woods, but in mine they didn't use sandstone for grinding stones, it put way to much grit in with their food. They had a lot of food that wore down their teeth as it was. I have been on some state digs and their teeth would be worn flat on the top of all their teeth from eating roughage all the time and grit from grinding stones. Sandstone depending on the type will wear out in no time if used very much.
Doesn't appear to have been used for grinding, bottom would be flat and smooth from use, and as monsterrak points out sandstone would have made their food real gritty.
I agree with others but would have picked it up too..... If nothing else it's different and will look good in the yard with the rest of my "rocks"- that's what the wife calls them
Also is ur username for Suzuki gsxr's?
If so I share your addiction