Formed at various periods in geologic history, some granites are among the oldest rocks known on the planet. Since they are formed deep inside the Earth, exposures of granite only develop when overlying rocks erode or the crust of the planet shifts. Granite has been mined for use as a paving and building stone since antiquity, and several deposits have been found that contain commercially important gemstones and minerals. The exact mineralogical composition of granites is relatively variable, but the rocks are usually classified based upon the relative abundance of plagioclase and alkali feldspar.