Found this the other day. It looks like a sliver of the base but I posted it to a facebook page and people are saying its either a button buck or a knee cap or bone of some sort. Anyone here know? Doesn't seem like a button since the smooth side is concave and the rough side looks more like the part of the skull that is permanent.
It's the bony growth a person gets years after they have an eye poked out. I believe that belongs to One Eyed Willie.