Mining is the extraction of geological materials and minerals from Earth's surface. Mining is required to obtain most materials that cannot be grown through agricultural processes, or feasibly made artificially in a laboratory or factory. Ores recovered by mining include metals, coal, oil shale, gemstones, limestone, chalk, dimension stone, rock salt, potash, gravel, and clay. The ore must be a rock or mineral that contains a valuable constituent, can be extracted or mined, and sold for profit. Mining in a wider sense includes extraction of any non-renewable resource such as petroleum, natural gas, or even water.