Selenium is a rare element with a particular unpleasant smell and occurs in the native state with sulfur in the form of seleniate in minerals such as eucairite (CuAgSe), claustalite (PbSe), naumanite (Ag2Se), crookesite ((CuTlAg)2Se) and zorgite (PbCuSe). The main selenium sources are copper ores, from which it is recovered as a by-product in the processes of electrolytic refining. The largest world producers are the USA, Canada, Sweden, Belgium, Japan and Peru.