Slate shingles cover the central dome and roof of the National Museum of Natural History. Slate makes good shingles because it splits easily into thin, durable sheets of solid rock. Why? It contains platelike mica minerals arranged in narrowly spaced layers. Several varieties of granite—a durable and beautiful igneous rock—face and trim the Museum. A bright green-and-orange granite, unakite, accents the Mall-side stairs. The green is the mineral epidote, and the orange comes from hematite inclusions in orthoclase.