I've seen coins like this before. The ones I've seen were laying around in a kitchen for a good while. The grease/smoke/steam or whatever else in the air and maybe some dust. The side facing up can turn a brownish, copperish, and greenish depending on the metal and contaminants.
If it is something on the surface, acetone should fix it.
Edited by Seeker_101
08/20/2024 3:43 pm