Ore is natural rock or sediment that contains desirable minerals, typically metals, that can be extracted from it. Ore is extracted from the earth through mining and refined, often via smelting, to extract the valuable element or elements. The grade of ore refers to the concentration of the desired material it contains. The value of the metal an ore contains must be weighed against the cost of extraction to determine whether it is of sufficiently high grade to be worth mining. Metal ores are generally oxides, sulfides, silicates, native metals such as copper, or noble metals such as gold. Ores must be processed to extract the elements of interest from the waste rock. Ore bodies are formed by a variety of geological processes generally referred to as ore genesis.