I think it's not the copper wearing off. Looks like it had help. Could have been heated with a lighter or something with all the bumps on it. I found one the other day, but the reverse was still plated normally but the center of the obverse was missing the plating. I couldn't see any evidence of heat applied to it, but it looks like a partial plating/missing plating. I could fix up an image if anyone is interested in looking at it. Probably Mike Diamond would know more on the subject.