Im an archeologist.....its a mortar stone for grinding grains. There would be another stone associated with it, probably long gone.

The hole there was a thin spot worn down to nothing, they either wore through it or it got so thin it broke out at some later date.

see ya/

BTW: you idiots and immature morons "sex stone" you are why our country is stupid and useless now. Read a fucking book ass holes, then do some physical labor.