Posted 10/04/2023 07:22 am
I believe that's some form of environmental staining--a light (not heavy) film of crud...perhaps once a liquid long ago, now dry for many, many years. For fun, try soaking the coin in 100% acetone--might (might) reduce or remove the 'film'. There's always the possibility, however, that the surfaces have been lightly etched--again, an environmental issue.
