Rock Cycle Diagram
Rocks are broadly classified into three groups: igneous, sedimentary and metamorphic, and the simplest diagram of the "rock cycle" puts these three groups in a circle with arrows pointing from "igneous" to "sedimentary," from "sedimentary" to "metamorphic," and from "metamorphic" to "igneous" again. There is some sort of truth there: for the most part, igneous rocks break down at the Earth's surface to sediment, which in turn becomes sedimentary rocks. And for the most part, the return path from sedimentary rocks back to igneous rocks goes through metamorphic rocks.