A major class of rocks formed by cooling and consolidation from the molten state. They are classified by their grain size and chemical composition. those that crystallised at depth are known as "Intrusive" or "Plutonic", those that hardened at or near the surface are called "Extrusive". Those rich in silica are sometimes called "Silicic Igneous Rocks", and those poor in silica are known as "Mafic Igneous Rocks". "Kimberlite", the source of diamonds, is a mafic igneous rock.