In geology, rock's a naturally occurring solid aggregate of minerals and/or mineraloids. The Earth's outer solid layer, the lithosphere,'s made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic. The scientific study of rocks's called petrology, and petrology's an essential component of geology. Rock classification » Rocks are classified by mineral and chemical composition, by the texture of the constituent particles and by the processes that formed the… (More on Rocks)