100% NOT a concretion with man made patterns around the rim. Sorry.
I beg to differ. That is a concretion. The lines you are talking may or may not be incised. They may also be natural.
These look almost identical to your piece except for the lines and some have what look like incised lines around them though.
I see the possibility of it being concretion WITH human inscribed design's on the rim. Still can't get a good pic of the design, but I am going to take it to a local expert, and will provide an update
We need more Cowbell!!
Sure it's possible. I would expect to see discoloration inside the bowl tho.Maybe a possibility?