Any ideas what this was used for?

[bout2break]

Found near creek. Both sides look like first pic and 2nd pic shows how it looks from side. When holding it thump and forefinger fit perfectly in depression area. Thought maybe it was used to start fires but then on side should be solid and not hollowed out on both sides The edges are smooth so not sure what it could be used for. Thanks for help.

 

[The Grim Reaper]

Natural stone formation. Not an artifact.

 

[kenley]

I agree.

 

[rock]

The water really put a nice polish on it. Interesting I would of carried it home.

 

[quito]

Cool stone.

I would have brought it home too.