Cobalt(II) oxide, also known as cobaltous oxide, is a chemical compound. Its chemical formula is CoO. It contains cobalt in its +2 oxidation state as an ion. It also contains oxide. It can be green or red crystals. It can also be a grey or brown powder. It can be made by reacting cobalt(II) chloride with a base. Then the cobalt(II) hydroxide is heated to make cobalt(II) oxide. It can react with acids to make salts containing cobalt. It is used to make blue glass.