Desert varnish, also called rock varnish, is a small miracle of resistant rocks. It occurs widely but is most conspicuous in arid environments. It is a brown to black patina, only nanometers thick, composed largely of clays combined with manganese and iron oxides, and it gives exposed rock faces, be they scattered cobbles on the desert ﬂoor or entire mountain or canyon sides, a distinctive, wind-burnished coloring. Where a high proportion of iron is present, the patina tends toward red; the more manganese dominates, the blacker the “varnish” becomes. The most remarkable characteristic of desert varnish is that it is alive. Until relatively recently, geologists thought that desert varnish was an artifact of weathering—a result of slow oxidation—but anomalies discredited that explanation: rocks that contained little manganese were found to bear desert varnish with high levels of manganese oxide. In time, researchers learned that certain bacteria with powerful abilities to capture, metabolize, and concentrate trace amounts of manganese from windblown dust colonize the often superheated and usually moistureless faces of varnished rocks. These microorganisms appear to be the principal agents in the slow and steady accretion of oxides. Moreover, the varnish forms at such an even rate that it can be used to determine the date of rockslides and other events that initiate the exposure of rocks to the elements. Sometimes that exposure is human-caused: for several thousand years, natives of the North American Southwest have hammered at rock surfaces to remove the desert varnish and reveal the lighter-colored rock beneath it. In this way they have etched images— petroglyphs—of inﬁnite import and variety into the landscapes of the region. While the “meaning” of much “rock art” remains a mystery, the date of many glyphs’ creation can now be determined by radiometric techniques that measure tiny bits of carbon in the varnish and, hence, the length of time the varnish has been forming.