Igneous rocks are formed when molten rock is either ejected directly on to the earth's surface (extrusive rock) or pushed up near the surface and cools beneath it (intrusive rock). Molten rock beneath the earth's crust is called magma; when magma is ejected through the crust it is called lava.
Crystals are generally large and visible to the naked eye. Intrusive igneous rocks are the source of many gemstones.
Kavanaugh, Pocket Naturalist
- Kavanaugh, James. Pocket Naturalist - Geology - An Introduction to familiar Rocks, Minerals, Gemstones & Fossils, Waterford Press: Arizona (2000)