"Igneous" comes from the Latin ignis, meaning "fire." Igneous rocks are formed by cooling magma, which is the molten rock in the Earth’s crust and consists of liquids, crystals, and gases. They therefore cannot include shells or fossils, which are classic markers of many sedimentary rocks.
Igneous rocks themselves may be subdivided, depending on how rapidly they cool when solidifying. The slowestcooling rock forms the larger crystals, while quickly cooling rock forms small crystals, thus: