It's called toning and is a form of tarnish that occurs when the coin reacts with oxygen (and some pollutants) in air. Toning is prized by many collectors because it can be quite beautiful and is...
Amulet Coin is a Hold Item, and Blue version did not have the capabilities of attaching Items to pokemon. Gold, Silver and Crystal were the first games to come up with the Amulet Coin, and you can...