Every rock is part of something called the rock cycle. All rocks take turns being different kinds of rocks. When a rock is so hot that it melts underground, we call that magma. If it EXPLODES out of a volcano, then we call it lava. When melted rocks finally cool off, we call them igneous (Egg - Knee - Us) rocks.
When a lot of rocks get broken and then put back together again as one big rock, we call that a sedimentary (Said - Him - In - Tree) rock. Those are made out of lots of little peices of rocks that are all STUCK together like glue.
The last one is special. When a rock is squeezed and cooked underground, it CHANGES into a metamorphic (Met - Uh - More - Fig) rock. Diamonds are metamorphic rocks! Oregon Caves National Monument is made out of a metamorphic rock called marble.
