Desert varnish is the thin red-to-black coating found on exposed rock surfaces in arid regions. Varnish is composed of clay minerals, oxides and hydroxides of manganese and/or iron, as well as other particles such as sand grains and trace elements. The most distinctive elements are manganese (Mn) and iron (Fe).
Bacteria take manganese out of the environment, oxidize it, and cement it onto rock surfaces. In the process, clay and other particles also become cemented onto the rock. These bacteria microorganisms live on most rock surfaces.
The sources for desert varnish components come from outside the rock, most likely from atmospheric dust and surface runoff. Streaks of black varnish often occur where water cascades over cliffs. No major varnish characteristics are caused by wind.
A complete coat of manganese-rich desert varnish takes thousands of years, so it is rarely found on easily eroded surfaces. A change to more acidic conditions (such as acid rain) can erode rock varnish. Lichens can also chemically erode rock varnish, as can visitors who scratch graffiti into it.