Gamblers' charms are often these dimes, Mercury being the Roman god who ruled the crossroads, games of chance, etc. Although these dimes actually figure the head of Liberty, people commonly mistake it for Mercury. A silver dime worn at the throat will supposedly turn black if someone tries to poison the wearer's food or drink.
People often believe that a dime with Liberty's head on it is a charm used by gamblers, but in reality, it's actually a dime featuring Mercury, the Roman god of crossroads and games of chance. Wearing a silver dime around one's neck is said to turn black if someone tries to poison the wearer.