A silver gray, brittle, hard metallic → chemical element which is highly magnetic; symbol Co. → Atomic number 27; → atomic weight 58.9332; → melting point 1,495°C; → boiling point about 2,870°C; → specific gravity 8.9 at 20°C. It is used in many → alloys, and in particular its compounds have been used since ancient times (Egyptians, Persians, Greeks) to produce a blue color in glass and ceramics. Cobalt was discovered in 1735 by the Swedish chemist Georg Brandt (1694-1768). It has several radioactive isotopes, including Co-56, half-life about 77 days, Co-57, 272 days, Co-58, 71 days, Co-60, 5.27 years. The → light curve of → type I supernovae is explained by the radioactive decay of nickel-56 through cobalt-56 to iron-56.
From Ger. kobold "evil spirits or goblins," who were superstitiously thought to cause trouble for miners, since the mineral contained arsenic which injured their health and the metallic ores did not yield metals when treated with the normal methods.