Which compound could be tarnish on a metal?

Tarnish on a metal is typically a compound formed by the reaction of the metal with sulfur or oxygen in the environment. For example, silver tarnish is primarily caused by the formation of silver sulfide (Ag₂S) when silver reacts with hydrogen sulfide (H₂S) in the air. Similarly, copper can tarnish to form copper oxide (CuO) or copper carbonate (CuCO₃) when exposed to moisture and air. These compounds create a dull, discolored layer on the surface of the metal.