Discovered in 1735, cobalt occurs most frequently as the sulfide in deposits of cobalt ores such as cobaltite, CoAsS, and smaltite, CoAs₂. Cobalt, like nickel and iron, is magnetic. The metal is obtained in greatest quantitity as a byproduct of the extraction of nickel, copper, and lead from their ores.

The metal itself is relatively unreactive. Exposure to high-temperature moist air gradually converts the metal to the oxide, CoO. This oxide is used to produce a deep blue color (i.e. what is known as "cobalt blue") in glasses, potteries, tiles, porcelain, ceramic glazes and paints.

Cobalt is used in some specialty alloys, alnico. Permanent magnets are made with such an alloy that contains 8% aluminium, 14% nickel, 24% cobalt, 3% copper, and 51% iron. Other alloys with chromium and tungsten are used for drill bits, lathe tools and surgical instruments. Some are used in making jet propulsion engines.

Many cobalt compounds are also used as contact catalysts for organic reactions. A radioactive isotope of cobalt is used to treat cancer. In 1935, it was established that traces of cobalt are essential to humans. Cobalt is present in meat, dairy products and in vitamin B₁₂, which prevents pernicious anemia. Cobalt is also needed for the growth of many animals. This element is normally supplied to the soil along with fertilizers.