Rock varnish, Drakensberg Mountains. View of desert varnish on eroded sandstone rocks. Desert varnish (or rock varnish) is a coating composed of clay minerals, manganese, iron and trace elements that forms on rocks in arid environments. It forms when wind-blown particles stick to wet rocks and then react in the heat of the sun, forming an accretion on the surface of the rock. Photographed in the Golden Gate Highlands National Park, Drakensberg Mountains, South Africa.