Rock varnish is a coating composed of clay minerals cemented to rock surfaces by oxides of manganese and iron. Although this dark brown-to-black accretion is most noticeable in arid regions, it occurs in all terrestrial weathering environments. Scholarly varnish research started with Alexander von Humboldt, when he asked how this external accretion forms and why manganese concentrations in varnish are 101–102 greater than in potential source materials. In the ensuing two centuries, investigations into rock varnish have been characterized by researchers studying only a handful of samples who have often used limited data to draw general conclusions. In contrast, nearly two decades of work by Tanzhuo Liu of Columbia University has yielded more than 10,000 varnish microstratigraphies obtained from rock depressions, analyses of which have provided new insights into the origin of rock varnish and the nature of climatic change in deserts, in addition to opening new research avenues in geomorphology and geoarchaeology.