Hmmm...ok, I think I jumped to conclusions here!
Now looking closer at your coin, do you notice how the spots have a consistently cubic shape, while varying in size?
You got my curiosity now...have you put this under a scope; does the discoloration appear to be differing color in the metal?
Seeing those cubic spots makes me almost think of some kind of crystalization at work.

I see the same process on the coin's rim.
I'm not a metallurgist--is it possible for different alloys to crystalize out of a cooling ingot in such patterns?