Desert varnish or rock varnish is an orange-yellow to black coating found on exposed rock surfaces in arid environments. Desert varnish is usually around one micrometer thick and represents nanometre-scale layering. Rock rust and desert patina are other terms which are also used for the condition, but less often. ==Formation == Desert varnish form...
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thin, dark red to black mineral coating (generally iron and manganese oxides and silica) deposited on pebbles and rocks on the surface of desert ... [2 related articles]
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http://www.britannica.com/eb/a-z/d/35
A hard, dark, shiny coating on rocks caused by chemical action.
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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.
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the dark, lustrous coating or crust, usually of manganese and iron oxides, that forms on rocks, pebbles, etc., when exposed to weathering in the desert. Also called
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