Sample of the element cobalt Co rotating. Cobalt has the atomic number 27, and is a hard transition metal. It is one of the few ferromagnetic metals, metals able to form a permanent magnet. It is used to make magnets, pigments, battery electrodes, magnetic recording media, diamond tools, high_speed steels, and cemented carbides. A radioactive isotope of cobalt, cobalt_60, is used as a medical tracer and in the treatment of some forms of cancer.