Macrophotograph of a sample of high purity 99, 8% electrolytic cobalt metal. The transition metal cobalt (Co). Cobalt is a hard, silver-white metal that is combined with other metals to make various alloys. 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. Horizontal object size of this section: 30.0 mm.