It cood be a Indian stone mabee for pouding or grinding a nut. Peoople say look for peckeing with a nother stone or anler. Maybee a litle peckeing marks?
I think it's a naturally shaped stone. The authentic ones will exhibit use wear as well as pecking and grinding you speak of. Got to study a lot to recognize them and know the tool-making techniques and material. To get you started, take a look at these images of...