Nickel(II) oxide is the chemical compound with the formula NiO. It is notable as being the only well characterized oxide of nickel (although nickel(III) oxide, Ni₂O₃ and NiO₂ have been claimed ). The mineralogical form of NiO, bunsenite, is very rare. It is classified as a basic metal oxide. Several million kilograms are produced in varying quality annually, mainly as an intermediate in the production of nickel alloys.