Desert varnish is a geological term for a dark and shiny coating on rock surfaces.
You can see it very clearly in the deserts in Arizona -- light colored rocks with a shiny lacquer-like surface on one side. Frequently you can find Native American petroglyphs, or modern graffiti, carved into the surface of a rock, so that the light rock shows through the scratches in the dark coating.
Nobody knows exactly how it forms, but the main theory is that it is from atmospheric dust that somehow reacts chemically with the rock surface over a long period of time. Manganese and iron oxides give the stuff its dark shiny color. It is also called rock varnish, since it doesn't only occur in deserts.