a silver-white metallic element with a faint pinkish tinge, occurring in compounds whose silicates afford important blue coloring substances for ceramics. Symbol: Co; atomic weight: 58.933; atomic number: 27; specific gravity: 8.9 at 20°C.

1675–85; < German Kobalt, variant of Kobold kobold Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
cobalt (ˈkəʊbɔːlt)
a brittle hard silvery-white element that is a ferromagnetic metal: occurs principally in cobaltite and smaltite and is widely used in alloys. The radioisotope cobalt-60, with a half-life of 5.3 years, is used in radiotherapy and as a tracer. Symbol: Co; atomic no: 27; atomic wt: 58.93320; valency: 2 or 3; relative density: 8.9; melting pt: 1495°C; boiling pt: 2928°C
[C17: German Kobalt, from Middle High German kobolt goblin; from the miners' belief that malicious goblins placed it in the silver ore]

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Word Origin & History

1680s, from Ger. kobold "goblin," Harz Mountains silver miners' term for rock laced with arsenic and sulphur (so called because it made them ill), from M.H.G. kobe "hut, shed" + *holt "goblin," from hold "gracious, friendly," complimentary words used to avoid the wrath of troublesome beings. The metal
was extracted from this rock. It was known to Paracelsus, but discovery is usually credited to Brandt (1733). Extended to a blue color 1835.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

cobalt co·balt (kō'bôlt')
Symbol Co
A metallic element, used chiefly for magnetic and high-temperature alloys and in the form of its salts for blue glass and ceramic pigments. Atomic number 27; atomic weight 58.9332; melting point 1,495°C; boiling point 2,930°C; specific gravity 8.9; valence 2, 3.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
cobalt   (kō'bôlt')  Pronunciation Key 
Symbol Co
A silvery-white, hard, brittle metallic element that occurs widely in metal ores. It is used to make magnetic alloys, heat-resistant alloys, and blue pigment for ceramics and glass. Atomic number 27; atomic weight 58.9332; melting point 1,495°C; boiling point 2,900°C; specific gravity 8.9; valence 2, 3. See Periodic Table.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Some wires, made of cobalt oxide and gold, become the negative poles of the
Shades of a cobalt blue fresco can be found above the stepped altar.
Beside them were battered cobalt blue trucks-the ones used to whisk away
  prisoners and detainees.
When a current runs through an electrode, phosphate and cobalt in the water
  form a thin film on that electrode.
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