From the August 2006 issue of Astronomy & Geophysics:

Desert varnish: an environmental recorder for Mars
Randall S. Perry and Mark A. Sephton
Astronomy & Geophysics 47, 4.34-4.35 (2006)
doi: 10.1111/j.1468-4004.2006.47434.x
Abstract

See also an earlier, related paper and press release:

Baking black opal in the desert sun: The importance of silica in desert varnish
Randall S. Perry et al.
Geology 34, 537–540 (2006)
Abstract

EurekAlert Release
Was there life on Mars? Shiny rock coating may hold the answer

That makes me feel a bit more comfortable about using the term "desert varnish" around Mars. The conventional wisdom has been that it was managnese and organics mediated by bacterial processes (not that I'm against finding life on Mars, but there are certain items that can be 'buzz words' around certain types ). If the coating can be explained by the simplest non-bio process it widens the envelope.


--Bill