Desert varnish is a dark, glossy coating found on the surface of rocks in arid environments, composed of clay minerals, oxides of manganese and iron, and organic material. It forms over long periods of time through the action of wind and water, and its study can provide insights into past environmental conditions and the history of the landscape. 

Reference: Dorn, R. I. (1998). “Rock Coatings.” Elsevier.