Not environmental damage. Not an altered coin. Not a counterfeit. What is it? A dime on zinc cent planchet, albeit broadstruck. (Kinda has to be since a cent planchet won't it fit in a dime collar.)
Awesome coin. I find it quite interest to see how much the surface metal moves as it is slayed out from the strike impact. The MM movement is way cool. The split plating shows more movement at the edge and less at the center. A feature that would not show up on a copper alloy cent. Is the edge void of all copper plating?
I have 1 Canadian quarter struck on a cent planchet. I love it... $425.00 I never regretted it. These go for way more money. I got a good deal on it. I collected toned caribou quarters from 1937-1967.
My fave off metal is the 1970 US quarter struck on a 1941 Canadian 25c coin that was sold a few years ago. Obviously a bored mint employee deliberately created it.