Cobalt is a metal element, symbol Co, atomic number 27. In the visual arts, this gray metal is the basis of blue and sometimes green pigments. It is normally found as a by-product of mining copper and other metals; it can also be found in the mineral colbaltite, which isn't very pretty.
Cobalt and kobold, a goblin, are related in name-- early workers found the gray residue after obtaining the copper or nickel they were after, and called it kobolt, explaining it as a kind of earth spirit that had found its way into the ore. So, fantasy artists, let's use a little cobalt pigment when painting your kobolds. It will be our little joke.
Combined into the structure CoAl2O4 it becomes the blue shade we are familiar with in painting, glass making, and white-and-blue Chinese porcelain.
Cobalt blue was a favorite of Maxfield Parrish, and is sometimes also called Parrish Blue. Cobalt green is a mixture of cobalt zincate and zinc oxide. Both pigments are very stable, and therefore good choices for a painter who wants their work to last a long time.
Some cobalt compounds are also used as catalysts that speed the drying of inks, paints, and varnishes.
Although cobalt is part of vitamin B12, it is toxic and should not be ingested outside of food/supplements, where it a small part of the compounds of life. Likewise, anyone working with pigment powders should wear a protective mask to avoid accidental inhalation. The LD50 can be assumed at over 10 grams, ingested, but it is carcinogenic, so it is still best to wash up carefully after using cobalt paints.
