I worked for a guy who had a large grindstone and grinding plate. He found the plate part in a creek and looked around a bit more and found the matching grinder part.
I always laugh at archaeologists who get all upset at people who keep found arrowheads and such. I tell them that if I find out who owned those things, of course I’ll return them. Until then, finders keepers, and if you want to study artifacts, get out there and find them yourself.
I don’t, however, approve of dealer types who will mine an entire site just to sell the stuff. If that’s inconsistent, too bad.