It looks like a mold for early metal tool making. In this case, the early tool makers would smelt a metal, pour it in a mold like that and cover it with another mold done just opposite. They would then tie the two together and wait for it to harden. All you have to do is pop it out and tie it on the end of a stick and instant axe.

Then again, it could just be a rock with a hole in it.