I have read in our old MAS (Massachusetts Archaeological Society) publications the designation of "Killed" artifacts. This one was found in a well-used agricultural field. And, though the field was repeatedly plowed for likely a hundred+ years, I can't imagine that it was broken like that by a plow blade.
I can see it getting knocked around, scratched and scarred up, but jeesh!
I did put a coat of mineral oil on it to bring out the color before adding it to your post. It was chalky and bluish before I did.
Also, as far as purpose, It seems a bit heavy to be hung around someone's neck. Unlike your thinned down version, this one is quite thick. Maybe it was just for the powwow. I do have one other Gorget that is whole. It is also thin (two pieces glued together, found on different hunts). Also, I don't think it's made of slate.