cobalt (n.)

1680s as the name of a type of steel-gray metal, from German kobold "household goblin" (13c.), which became also a Harz Mountains silver miners' term for rock laced with arsenic and sulfur (according to OED so called because it degraded the ore and made the miners ill), from Middle High German kobe "hut, shed" + *holt "goblin," from hold "gracious, friendly," a euphemistic word for a troublesome being.

The metallic element (closely resembling nickel but much rarer) was extracted from this rock. It was known to Paracelsus, but discovery is usually credited to the Swede George Brandt (1733), who gave it the name. Extended to a blue color 1835 (a mineral containing it had been used as a blue coloring for glass since 16c.). Compare nickel. Related: Cobaltic; cobaltous.