Andesite, a fine-grained rock that is widely distributed in the earth's crust. Andesite reaches the surface as lava from volcanoes. As it cools it forms gray to grayish-black sheets, often in thick accumulations. Andesite consists chiefly of feldspar minerals mixed with smaller amounts of iron and magnesium. It was named for the Andes of South America, where it is widely found. Andesite is used for building stone and monuments.