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Rocks are mixtures of one or more minerals. Sedimentary rocks are made of solid, loose pieces of rocks (in the form of sand, silt, clay, and gravel), or the remains of living things found at the surface of the Earth, that over a very long period of time, has become compacted and cemented into sedimentary rock. Igneous rocks come from melted rock material, or magma, that lies beneath Earth’s surface. Metamorphic rocks are rocks that have become changed by intense heat or pressure while forming.