@coin, first welcome to CCF. Second, if this coin was clad, then I'd tell you that we sometimes see this on coins exposed to high heat. For a solid silver alloy, I'm not sure if the same thing might happen. 
"If you climb a good tree, you get a push."
-----Ghanaian proverb
"The danger we all now face is distinguishing between what is authentic and what is performed."
-----King Adz
-----Ghanaian proverb
"The danger we all now face is distinguishing between what is authentic and what is performed."
-----King Adz





















