Granite is usually a light-coloured, coarse-grained igneous rock consisting mostly of quartz, orthoclase feldspar, sodium-rich plagioclase feldspar, and micas. Quartz usually makes up 10 to 50 percent of the light-coloured minerals in granite, with the remaining minerals consisting of feldspars and muscovite. The darker minerals in granite are usually biotitic and hornblende. Granite is one of the most common rocks in the crust of continents, and is formed by the slow, underground cooling of rocks in the crust of the earth, and is formed by the slow, underground cooling of magma.