Tarnish:Tarnish is a thin layer of corrosion that forms on the surface of metals. It is caused by exposure to oxygen and moisture, and can occur naturally or be accelerated by chemicals or pollutants in the air. Tarnish is usually a dull, brown or black color, and can make jewelry and other metal objects look old and dirty. While it is not harmful to the metal itself, it can be difficult to remove once it has formed.