Desert varnish

[4][5] However, a 2008 microscopy study posited that desert varnish has already been reproduced with chemistry not involving life in the lab, and that the main component is actually silica and not clay as previously thought.

[8] Clay, then, acts as a substrate to catch additional substances that chemically react together when the rock reaches high temperatures in the desert sun.

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.

Shiny, dense and black varnishes form on basalt, fine quartzites and metamorphosed shales due to these rocks' relatively high resistance to weathering.

Its presence has been cited as a key factor in the preservation of a large number of petroglyphs dating back to the Iron Age and earlier in areas such as the Wadi Saham in the United Arab Emirates.