Cobalt, Symbol Co , a hard, brittle metallic element, found associated with nickel, silver, lead, copper, and iron ores and resembling nickel and iron in appearance. It is used for magnetic alloys, high-temperature alloys, and in the form of its salts for blue glass and ceramic pigments. Atomic number 27;  melting point 1,495°C; atomic weight 58.9332; boiling point 2,900°C; valence 2, 3; specific gravity 8.9.

The element is chemically active, forming many compounds, the series of cobaltous and cobaltic salts and the complex cobalt ammines derived from cobaltic salts and ammonia. Cobalt  green, yellow, and blue are pigments of high quality that contain cobalt; another blue pigment, smalt, is made by powdering a fused mixture of cobalt oxide, potassium carbonate, and sand; these pigments are often used for coloring ceramics and glass. Cobalt chloride, used as an invisible ink, is almost colorless in dilute solution when applied to paper. Upon heating it undergoes dehydration and turns blue, becoming colorless again when the heat is removed and water is taken up.

Cobalt-60 (60Co) is a radioactive isotope of cobalt, with a half life of 5.27 years. 60Co decays by negative beta decay to the stable isotope nickel-60 (60Ni). In the process of decay, 60Co emits one electron with an energy of up to 315 keV and then two gamma rays with energies of 1.17 and 1.33 MeV, respectively.

One gram of 60Co contains approximately 50 curies (1.85 terabecquerels) of radioactivity. With a point source at a range of 1 m, this amount of 60Co would give a person a radiation dose of 0.5 sievert (50 rem) per hour. 60Co has seven main beneficial uses:

• Sterilization of medical equipment,
• As a tracer for cobalt in chemical reactions,
• Radiation source for industrial radiography,
• Radioactive source for leveling devices and thickness gauges,
• Radiation source for medical radiotherapy,
• As a radioactive source for food irradiation and blood irradiation, and
• As a radioactive source for laboratory use.

sources : wikipedia, thefreedictionary