I'll never quite understood how every coin tones differently. This coin seems rather odd. It's been in this holder for as long as I can remember and the other coins are barely toned .
Take them beauties out of that holder @furham. The discoloration appears to be environmental (moisture) mainly from not a good seal. It happened to me. Just my opinion. Good luck.
I can clearly see the same mark on the right end of the holder, so something is contaminating the area.
