According to their origin or process of formation, rocks can be classified into three groups: igneous or magmatic, sedimentary and metamorphic.

such as granite and syenite, are formed by the slow cooling of magma under the Earth's surface. This can take thousands or even millions of years, which allows for larger crystal formation.

they are formed when magma hardens close to the surface in cracks in the Earth's crust after a period of slow cooling deep inside the Earth. This allows some parts of the magma time to crystallise, while others do not. Porphyry and pegmatite are examples of them.

Volcanic rocks are formed when magma reaches the Earth's surface without cooling. Outside, the magma cools and hardens rapidly. The resulting rock has either very small crystals or no crystals at all. Basalt, gabbro, obsidian and pumite are volcanic rocks.

are formed by the settling, accumulation and consolination of sediments or by the precipitation of minerals from a solution.

Conglomerate is made up of rounded rock fragments cemented together. The pebbles have been rounded by the action of moving water. The size of these rock fragments is over 2 mm in diameter.

Breccia is very similar to conglomerate but the fragments that make it up are angular. The size of these rock fragments is over 2 mm in diameter.

Sandstone is made of sand grains cemented together into solid stone. The grain size of sandstone is 0.02 to 2 mm in diameter. Sandstone is mostly made up of quartz.

Clays and mudrocks are made of very fine particles (diameter under 0.02 mm).

Carbonates, such as limestone, are formed by the precipitation of calcium carbonate dissolved in water.

Evaporites are formed when the water from a salt solution evaporates, leaving solid salt behind. Examples: gypsum and halite or salt rock.

Biochemical sedimentary rocks form from the gradual accumulation of the remains of living things, such as shells or dead plant material. They include shelly limestone, coralline limestone, coquina, coal and crude oil.