It might be paint, glue, nail polish, varnish, lacquer, or what else else I missed. An alteration on a coin does not improve the value. More like if you had a new car and you hit it on the fender with a sledge hammer and made a dent on it. Would that improve the value of your car? No, it is just damaged. Same on coins; If you alter the finish, then they are less desirable to a collector. But if it was struck that way from the mint, it might improve the value. But sometimes done to a coin, couldn't of happened at the mint. Knowing what caused the issue on a coin will help you to determine what happened to your coin. So damage/alteration/re-processing, re-plating only keeps their value to just face value. (if you can spend them if not take them to the bank to be sent back to the mint)