Desert varnish is a dark coating on rocks in arid regions. It often coats canyon walls where water has flown down the the sides of canyons. The coating is composed mainly of fine-grained clay minerals and within the clays are black manganese oxide and red iron oxide which create the different colors.
When hiking in areas, particularly in southern Utah and northern Arizona, I've often come upon areas where desert varnish has covered a rock face like a beautiful painting. Please feel free to add your own photos of desert varnish!