The common metals, excepting copper, do not occur free in nature, but are produced from their ores which generally require chemical treatment at high heat in furnaces. Uncombined copper, known as "native" copper, mixed with more or less earthy material, is found in some parts of the world, notably in the Lake Superior region, but the main source of all engineering metals is their ores, which are more or less abundantly distributed over the earth.

An ore is a natural substance composed of a metal chemically combined with one or more non-metallic substances. Ores of different metals are sometimes found incorporated in the same mass, and almost all ores are found mixed with impurities of a non-metallic nature, as rock, sand, clay, etc., known in mining as gangue. Many ore deposits within range of easy transportation cannot be profitably worked either because of excessive gangue or because of chemical composition of ore or gangue which renders smelting unprofitable.