Cobalt Metal, steel gray metal, chemically stable at room temperature, will oxidize to form CoO when heated above 300 ℃ in air, and burn to form Co3O4 when white hot. The melting point is 1495 ℃, the boiling point is 2870 ℃, and the hardness is high. Stellite alloy containing over 50% cobalt still maintains its hardness at 1000 ℃.