Cobalt it reacts slowly with mineral acids, and very slowly with moist, but not with dry, air. Common oxidation states of cobalt include +2 and +3, although compounds with oxidation states ranging from −3 to +5 are also known. A common oxidation state for simple compounds is +2 (cobalt(II)). These salts form the pink-colored metal aquo complex [Co(HO)] in water. Addition of chloride gives the intensely blue . In a borax bead flame test, cobalt shows deep blue in both oxidizing and reducing flames. Several oxides of cobalt are known. Green cobalt(II) oxide (CoO) has rocksalt structure. It is readily oxidized
Cobalt ruins of Pompeii, destroyed in 79 AD, and in China, dating from the Tang dynasty (618–907 AD) and the Ming dynasty (1368–1644 AD). Cobalt has been used to color glass since the Bronze Age. The excavation of the Uluburun shipwreck yielded an ingot of blue glass, cast during the 14th century BC. Blue glass from Egypt was either colored with copper, iron, or cobalt. The oldest cobalt-colored glass is from the eighteenth dynasty of Egypt (1550–1292 BC). The source for the cobalt the Egyptians used is not known. The word "cobalt" is derived from the German "", from "kobold" meaning