Rock or stone is a natural substance, a solid aggregate of one or more minerals or mineraloids. For example, granite, a common rock, is a combination of the minerals quartz, feldspar and biotite. The Earth's outer solid layer, the lithosphere, is made of rock. Rock has been used by mankind throughout history. The minerals and metals found in rocks have been essential to human civilization.
- Roberts, Dar. "Rocks and classifications". Department of Geography, University of California, Santa Barbara. http://earth.rice.edu/mtpe/geo/geosphere/topics/rocks_a.html. Retrieved 11 November 2012.