Toning (coin)

Toning on a coin is the change of colour brought about through oxidization, which forms a thin layer of tarnish on the metal's surface, typically as a result of interactions and chemical reactions with sulfur-containing compounds in the environment.^[1][2] Coin toning can vary in aesthetics, and thus can alternately add or detract from a coin's value.^[3] Coins with unusual toning colours or patterns tend to be valued by coin collectors. Toning is most common on silver coins, but can be found on coins minted from other metals and alloys, including gold, copper and cupronickel.