Desert varnish on cliffs of massive De Chelly Sandstone in Canyon de Chelly National Monument, Arizona. Desert varnish is a thin, dominantly manganese mineral coating that forms on the surface of rock. Manganese-fixing bacteria living on the rock surface help trap minerals from atmospheric aerosols (clays, dust) and streaming water (Dorn and Oberlander, 1982; Garvey and others, 2008; also, click here to see a NPS website about desert varnish.).