Desert Varnish
(c) 2000 Andrew Alden, licensed to About.com, Inc. (fair use policy)
Both rocks in this image consist of Pinto Mountain Gneiss, a tough quartz-rich stone the color of pinto beans. But the stone on the right is covered with desert varnish or rock varnish.
Desert varnish is composed of clay minerals from local sources, plus black manganese minerals such as birnessite, a hydrated manganese oxide. It appears that after thousands of years of surface exposure, manganese-oxidizing bacteria living on the rock surface build up the dark layer, which is no thicker than a sheet of paper.
The longer a rock is exposed, the darker desert varnish gets. In flat areas, the whole desert floor can be darkened, but on the scree slope in southern Joshua Tree National Park where I took this picture, fresh rock and old rock are jumbled together. The desert varnish is chipped and flaked where other stones have struck it.
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