In intrusive igneous rocks, the crystals are typically large enough to be seen with the naked eye. This size is the result of the slow cooling of magma beneath the Earth’s surface, which allows the minerals to crystallize and grow into larger grains. Generally, the crystal sizes in these rocks can range from a few millimeters to several centimeters in diameter.
For instance, granite, a common type of intrusive igneous rock, consists of large crystals of quartz, feldspar, and mica. The visible crystal size provides important information about the cooling history of the magma; slower cooling results in larger crystals, while faster cooling, as seen in extrusive rocks, leads to smaller crystals or even a glassy texture.