I believe it is natural. Where I live stones like that are still being formed in the rapids at the fall line of the James River (Richmond, VA). What happens is a stone rolling down river gets caught up in the rapids just right to get rolled around and around, like it's stuck in a washing machine. The result is a round rock and a reamed out, bowl like depression left in the big granite slabs that make up the rapids. Many years ago I had a neighbor that regularly walked the river and would find them, especially during droughts when the water was low. Nearly perfectly round like one shown here are rare, though. I have another friend that has one a little bigger than that he picked up in the river, made of granite, but not perfectly round. Most of them I've seen from around here are quartzite cobbles that are already round, then get caught in mother nature's rock tumbler in the rapids and get rounder and more polished.