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Perhaps because its ability to alter a landscape’s appearance dramatically (Fig. 8.1), the literature on rock varnish remains one of the largest in the general arena of rock coatings (Chapter 10 in Dorn 1998). Rock varnish (often called "desert varnish" when seen in drylands) is a paper-thin mixture of about two-thirds clay minerals cemented to the host rock by typically one-fifth manganese and iron oxyhydroxides. Upon examination with secondary and backscattered electron microscopy, the accretionary nature of rock varnish becomes obvious, as does its basic layered texture imposed by clay minerals (Dorn &Oberlander 1982).

Mostly calcium oxalate and silica with variable concentrations of magnesium, aluminum, potassium, phosphorus, sulfur, barium, and manganese. Usually dark in colour, but can be as light as ivory Fig. Geological Survey Professional Paper 475-B, B28-B31.

8.1 This road cut between Death Valley, California and Las Vegas, Nevada illustrates the ability of 10-100µm-thick rock varnish (darkens debris slope surface on upper half of image) to mask the appearance of the host igneous and metamorphic rocks (light-appearing road cut on lower half of image). (1963) Variation in minor-element content of desert varnish.