Can some one smarter than me fill me in on what is going on with this penny

Need help can't find any info on what would cause this.

 

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Circulation did that.

 

OK I could see that if it was zinc but how can it change copper to grey. I've been trying to google to find a chemical reaction but come up empty.

 

science class dipping/plating,

 

any 1 or a combination of a thousand or more environmental chemical reactions could cause that damage...

 

I have found this experiment online but have yet to find it were only one side of coin is affected. When I magnify it there does not appear to be any coating as well as on edges and around to reverse copper seams to be on top. I don't have a good scale all I have is a balance and when placed against any known copper penny it is slightly lighter. I found ways to reverse said experiment so if I can't find anything for certain I am going to do that

 

in the long run, its damaged.