Most rocks are a mixture of many different minerals and contain naturally occurring compounds. There are THREE main types of rock.
Sedimentary rocks are usually quite soft and crumbly, this is because they are made of grains that are stuck together. They are formed in layers when sediment is deposited. Shown below are two examples of sedimentary rocks.
Metamorphic rocks are usually hard, smooth and shiny and may have layers of crystals that give them a glittery appearance. They are made of interlocking crystals which are formed when rock is changed by high amounts of pressure and heat.
Igneous rocks are very hard and similar to metamorphic rock, are formed by interlocking crystals. They are formed when molten rock cools and solidifies. The size of the crystals depends on the rate of cooling.