Tarnish is a thin layer of oxidation or other chemical reaction product that forms on the surface of a metal. The specific compound will depend on the metal in question. Here are some common examples:

* Silver: The most common tarnish on silver is silver sulfide (Ag₂S), which is a black, powdery substance formed by the reaction of silver with sulfur in the air.

* Copper: Copper tarnish can be a variety of colors, including copper oxide (CuO), which is black, and copper carbonate (CuCO₃), which is green.

* Brass: Brass, an alloy of copper and zinc, tarnishes similarly to copper, forming copper oxide and copper carbonate on the surface.

* Iron: Iron rusts, which is a form of iron oxide (Fe₂O₃). While rust isn't technically "tarnish," it serves a similar purpose in protecting the underlying metal.

So, depending on the metal, the compound forming the tarnish could be a sulfide, oxide, carbonate, or other reaction product.