Cobalt Cobalt Cobalt is a chemical element with symbol Co and atomic number 27. Like nickel, cobalt is found in the Earth's crust only in chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal. Cobalt-based blue pigments (cobalt blue) have been used since ancient times for jewelry and paints, and to impart a distinctive blue tint to glass, but the color was later thought by alchemists to be due to the known metal bismuth. Miners had long used the name "kobold ore" (German
Cobalt with water and oxygen to brown cobalt(III) hydroxide (Co(OH)). At temperatures of 600–700 °C, CoO oxidizes to the blue cobalt(II,III) oxide (CoO), which has a spinel structure. Black cobalt(III) oxide (CoO) is also known. Cobalt oxides are antiferromagnetic at low temperature: CoO (Néel temperature 291 K) and CoO (Néel temperature: 40 K), which is analogous to magnetite (FeO), with a mixture of +2 and +3 oxidation states. The principal chalcogenides of cobalt include the black cobalt(II) sulfides, CoS, which adopts a pyrite-like structure, and cobalt(III) sulfide (CoS). Four dihalides of cobalt(II) are known: cobalt(II) fluoride (CoF, pink), cobalt(II) chloride (CoCl,