A rock is a consolidated mixture of the same or different minerals. By consolidated, we mean hard and strong; real rocks don’t fall apart in your hands. A mixture of minerals implies the presence of more than one mineral grain, but not necessarily more than one type of mineral. A rock can be composed of only one type of mineral (e.g., limestone is commonly made up of only calcite), but most rocks are composed of several different minerals (e.g., the pegmatite in Figure 6.1). A rock can also include natural materials not classified as minerals, such as organic matter within a coal bed, or volcanic glass.
Rocks are grouped into three main categories based on how they form. Igneous rocks form from the cooling and crystallization of melted rock. Sedimentary rocks form when weathered fragments of other rocks are buried, compressed, and cemented together, or when minerals precipitate from solution, either directly or with the help of an organism. Metamorphic rocks form by alteration of a pre-existing rock under high heat and pressure. Although temperatures can be very high, metamorphism does not involve melting of the rock.