The rock cycle the process of cycling rocks involving sedimentary, igneous and metamorphical processes.
Calcium in the shells of dead sea animals gets buried under the sea. It is heated and recrystallized. This turns it into metamorphic rock e.g. slate. This rock is then melted and it turns into magma.
Rock only changes if it moves around. For example igneous rock is stable as long as it remains deep underground but if it moves to the earth's surface, where the temperature is lower and it is surrounded by oxygen, water and weak acids, it can be changed by breaking down.
The rock cycle is driven by energy; usually in the form of heat.
Big crystals show a rock cooled slowly from magma deep underground and small crystals show it cooled quickly on the surface.