The first individual piece needs better photo looking straight down on both sides of grove with something to tell what size it is. Looks like that piece has been ground into shape.
I guessing that they're natural too- but boy, I sure would have given them a good hard look, and I might have taken them home too! Why don't you tell us about where you found them. That might help us to explain why they look the way they do so you know better how to look for something awesome like a hammer
I see a 3 quarter groove, that being said could very well be natural. Please show the ends on the one you have two pics of so we can see if there is any wear?