Desert varnish

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What Wikipedia says

Desert varnish or rock varnish is an orange-yellow to black coating found on exposed rock surfaces in arid environments. Desert varnish is approximately one micrometer thick and exhibits nanometer-scale layering. Rock rust and desert patina are other terms which are also used for the condition, but less often.

Excerpt from the Wikipedia article “Desert varnish”, under the CC BY-SA 4.0 license. Read the article on Wikipedia

Image: Jason Hollinger, CC BY 2.0, via Wikimedia Commons.