Desert varnish is a surface stain, coating bedrock surfaces as well as surface sediments in arid regions around the world. The desert varnish itself comprises a sedimentary formation containing internal microscopic laminations. The chemical and mineral composition of each layer reflects the climate conditions under which it was deposited. A detailed study of these individual layers is used to both reconstruct local and regional climate history as well as quantitatively date geomorphic surfaces. Although the mineral and chemical components comprising desert varnish consist of wind-blown dust, atmospheric aerosols and microbes, the role of microbial activity in the formation of desert varnish remains a source of active research and lively debate.