"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 distinctive elements are Manganese (Mn) and Iron (Fe). The color of
rock varnish depends on the relative amounts of manganese and iron in it...Varnish surfaces tend to be shiny when the varnish is smooth and rich in manganese.
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.
Thousands of years are required to form a complete coat of manganese-rich
desert varnish so it is rarely found on easily eroded surfaces. "