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In black desert varnish, however, manganese is 50 to 60 times more abundant.

One proposal for a mechanism of desert varnish formation is that it is caused by manganese-oxidizing microbes (mixotrophs) which are common in environments poor in organic nutrients.

Desert varnish often obscures the identity of the underlying rock, and different rocks have varying abilities to accept and retain varnish.

Limestones, for example, typically do not have varnish because they are too water-soluble and therefore do not provide a stable surface for varnish to form.

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A micro-environment p H above 7.5 is inhospitable for manganese-concentrating microbes.

In such conditions, orange varnishes develop, poor in manganese (Mn) but rich in iron (Fe).

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Dark surficial coatings on rocks in diverse environments have been an object of scientific curiosity since their first description by Alexander von Humboldt (1819).

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