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Manganiferous rock varnish (desert varnish) is common in subtropical deserts, but coatings of amorphous silica glaze dominate rock surfaces of the same lithology and exposed to similar amounts of annual precipitation in the rain shadows of the large tropical shield volcanoes such as those in Hawaii. Lacking an explanation for this contrast, a two decade-long laboratory experiment tested importance of high water vapor content in silica glaze formation. Twenty years of exposing basalt rock chips to 80% and 90% levels of relative humidity revealed that water vapor alone can generate silica glaze and may be an important factor in explaining why silica glaze is the dominant rock coating in humid warm drylands. In addition to adding a new dimension to the formation of terrestrial rock coatings, these findings may have importance in interpreting any coatings of amorphous silica found on Mars.