a stone tool

[hudson]

just a small grinding stone made of iron ore sand cap.

[Ron Kelley]

Thanks for showing. It sure looks like it would do a good job of grinding.

[Hoss]

I have heard them called Pitted stones, Nutting stones and Cup stones. You can see a couple behind the anvil stone in this picture.

The depression in the anvil stone is perhaps 6 inches across for scale of size

[G10+]

Here is an other:-)