I see what your talking about it looks square in the pic but nothing up there in the round area, there is no noticeable chips like it was hand made etc. In fact the whole texture feels rough if that helps lol
I don't think this is anything organic (i.e., fossil/bone/tooth/etc), but the way you can verify that is pretty simple. Put your tongue on it. If there is no suction, it probably isn't bone. Being that this piece is jagged but not flaked, I don't think it is "worked", either. It looks like a rock that has been weathered by things like water as well as being broken/cracked.