The three main types, or classes, of rock are sedimentary, metamorphic, and igneous and the differences among them have to do with how they are formed. Sedimentary. Sedimentary rocks are formed from particles of sand, shells, pebbles, and other fragments of material.Together, all these particles are called sediment. Gradually, the sediment accumulates in layers and over a long period of time hardens into rock.edimentary. Sedimentary rocks are formed from particles of sand, shells, pebbles, and other fragments of material. Together, all these particles are called sediment. Gradually, the sediment accumulates in layers and over a long period of time hardens into rock.