I see what you mean with the stone NatureBoy, I think it could also be some sort of parasite or something? I remember reading something about a certain creature living in the mouth of fishes' replacing its tongue. I do not know if this is the case but the damage appears to be similar to that of the thing I read.
Edit: I might be wrong, I found some articles relating to this subject and everyone showed this happening to fish that we see in the market. So I guess I'm wrong on this one. Someone with more knowledge may have some input on this.