Scientists have developed a quick, easy, portable, and nondestructive way to determine the age of desert varnish, the mysterious dark coating that slowly develops on rocks in many arid regions of the world (see “Thin Skin,” in this week’s issue: Available to subscribers at Thin Skin). Desert varnish is rich in iron oxides and manganese oxide–both possible products of biological processes–and contains varying amounts of clay, says Nicholas E. Pingitore Jr. of the University of Texas at El Paso. Using varnish samples of known ages, Pingitore and a colleague have developed an X-ray–fluorescence technique that measures the iron and manganese in desert varnish and thus indicates its age.