People that put together electronic kits have been known to clean the head of the soldering iron on copper cents and I'd say that's what you have here, IMHO.
Ya only get 'blasted' if ya come 'round' here with an all knowing attitude, Felix. Yours was a question.....an inquiry.
Sometimes a coin used as part of jewelry looks like that when it's separated from the jewelry. Only the person who did it can tell you for sure.
Yep, solder. Sometimes epoxy is used. If the low temp silver solder is used the coin may not be discolored. Used in the wire fabrication industry and by me many times at Laribee Wire Co. Camden NY. How about that @ToppCatt ?