2009 Penny Strange Find

first thought is maybe the reverse spent lots of time on the ground causing it to change color

then the obverse would be exposed to other elements, resulting in another type of environmental aging.

Unless this was in a highly acidic sort of coin holder/forlder, then I think it was done deliberately.

Freesouvy,

Have you ruled out something being painted on the reverse? Nail polish or similar? Or is it definite toning?

I once opened a bottle of Lincoln cents that had been closed since 1981 - this was about 2014. I poured out a few of the bright, late seventies and arranged them on a carpeted area where I could see what I had. I was called away for something and when I got back to the cents, some of them had picked up a light, purple toning. Just withing an hour.

Nobody here could figure out what that was all about and I was a nub so nothing came of it. My feeling is that it was chemicals in the carpet where responsible. That my experiene. We may never know exactly what went on with your coin, bet keep researching.

Good guess Newbie.

We will never know the for sure answer but that makes sense.