Rocks are made of minerals. For example, the igneous rock basalt, which makes up most of the Earth's oceanic crust, is made of the minerals feldspar and pyroxene. Some minerals, such as sulphur and gold, are single elements (substances in which all the atoms are the same kind). Others are compounds of two or more elements. Feldspar is a compound of oxygen, silicon and aluminium with various other elements. The largest group of rock-forming minerals is the silicates, all of which include the elements silicon and oxygen. Quartz (the commonest mineral in the Earth's crust) is a silicate. The minerals inside a rock usually form small crystal grains that are locked together to form a hard solid.