When snails, corals, shellfish, and marine microorganisms die, their empty exoskeletons, rich in calcium carbonate or silica, pile up at the bottom of oceans and seas. The shells break up or decay into small pieces, called bioclasts or biochemical sediments. Over long periods of time, the upper layers of debris compress the lower layers, squeezing out excess water or air trapped between the shell fragments and breaking them down even further. Eventually, silica or calcite cements the individual fragments together to form a rock. The chalk you use to draw on the sidewalk comes from the shells of microorganisms that lived millions of years ago.

Humans have long prized granite, an igneous rock rich in the minerals quartz and feldspar, as a building material because it does not crack easily and can be polished. The Smithsonian's National Museum of Natural History building was built out of four different types of granite from four different states on the East Coast of the United States. Other rocks that make up the building include roof tiles of slate, and museum floors made of terrazzo, which is crushed marble and granite with cement filler. Elsewhere in Washington, D.C., marble, granite, and bluestone gneiss went into the Washington Monument; marble was used inside and outside the Lincoln Memorial; and red sandstone makes up the Smithsonian Castle on the other side of the National Mall. The nearby National Archives, Department of Justice, and Internal Revenue Service buildings have limestone facades and granite foundations.