A rock is defined as an aggregate of mineral grains, which means “a bunch of mineral grains all stuck together.” The mineral grains may be large enough to see with the naked eye (phaneritic) or microscopic (aphanitic). A granite specimen clearly shows its minerals: feldspar, quartz and often mica or hornblende. A basalt specimen is also composed of mineral grains, but these are too small to see. A rock may be an aggregate of grains of all the same mineral. Limestone is an example; it is made entirely from grains of the mineral calcite.

The three types of rocks are igneous, sedimentary and metamorphic. Igneous rocks form when molten rock, or magma, cools and hardens. Sedimentary rocks result from erosion of any rock type, followed by depositing the resulting sediment into a natural basin, and finally cementing the sediment into stone. Metamorphic rocks form when any rock is subjected to great heat and pressure, but not enough heat to melt the rock.

For example, when magma crystallizes, it may form the igneous rock, granite. If the granite is then eroded, it may become sand. Later, the sand may harden to form sandstone, a sedimentary rock. If the sandstone is heated and pressurized, it might turn into quartzite, a metamorphic rock. It is possible (although rare) for that quartzite to melt and then crystallize, turning it back into an igneous rock. This process can change any rock type into any other. It is called the Rock Cycle.