If you kick a dark pebble in the middle of the desert, you will sometimes find that it is light underneath. What this means is that you have disturbed a pebble that has been sitting there untouched for thousands of years. During that time, it accumulated a thin coating of windblown gunk - mainly clay dust, and manganese and iron oxides - known as desert varnish.
Desert varnish is not difficult to scratch through, and petroglyphic sgraffito is a popular artistic medium for native desert-dwellers and idiot tourists alike.
Desert varnish has a complex internal structure; there are thin…
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