desert varnish

n. thin and shiny layer that is reddish brown or black in color composed of iron and manganese oxides which coats the surfaces of many desert rocks that are exposed

Desert varnish, or rock varnish is a dark coating found on exposed rock surfaces in arid environments. It is one of fourteen different types of rock coatings.
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A thin, shiny red-brown or black layer, principally composed of iron manganese oxides, that coats the surfaces of many exposed desert rocks.