I will admit to not being an expert, but I have/had coins get that way from long exposure to just being out in the air, and air is full of dust particles.
That coin sure looks like it came in contact with some kind of chemical and now "environmentally damaged" What is a "rate proof"? And you must have been reading other new members posts from the past - LOL!!!
The famous 'Jimmy Durante' variety. Environmental damage......probably spent part of his life in the ground for a spell. Welcome to the forum young lady.
Welcome to the neighborhood, @ljc4457 ! It's either environmental or circulation damage. Me, I hate to dust! Chris