Among coin collectors, we call the process “toning” instead of tarnishing (a much more benign word, IMHO ;) It’s the exact same chemical process, most commonly

Among coin collectors, we call the process “toning” instead of tarnishing (a much more benign word, IMHO ;) It’s the exact same chemical process, most commonly seen in old silver dollars that were stored in sulpher-containing mint/bank bags. Just the right amount of toning can turn a silver-colored silver coin into a rather magnificent rainbow that can be worth many many times the value of the exact same coin without the coloring. Exhibit A: Regular silver dollar (still quite beautiful!) https://images.pcgs.com/CoinFacts/25003275_37921476_2200.jpg https://images.pcgs.com/CoinFacts/25003275_37921476_2200.jpg Exhibit B: Rainbow toned silver dollar that can be anywhere from 3-100x the price of a non-toned coin: https://images.pcgs.com/CoinFacts/35923065_149932537_2200.jpg https://images.pcgs.com/CoinFacts/35923065_149932537_2200.jp... As one would expect, values like this have created quite the cottage industry of artificially-toned coins which can be anywhere from trivial to impossible to differentiate from naturally-toned coins. It can be somewhat of a cat and mouse game to detect :)