Tarnish

Tarnish is a layer of corrosion similar to rust. It is caused by oxidisation - when semi - reactive metals such as copper, brass, silver, or aluminum meet chemicals in the air. It will cause the metal to darken or go dull by making a thin film on the metal. It is caused by a chemical reaction, between a metal and a non metal (or compound), usually oxygen or sulphur dioxide.

Tungsten rods with evaporated crystals, partially oxidized with colorful tarnish

A Morgan dollar demonstrating a colourful form of toning on its reverse. While tarnishing on other metal objects is generally cleaned off, some toning on coins may be considered aesthetically pleasing or beneficial to the coin's value, and is therefore kept in situ.