Nickel(II) oxide is the chemical compound with the formula . It is the principal oxide of nickel. It is classified as a basic metal oxide. Several million kilograms are produced annually of varying quality, mainly as an intermediate in the production of nickel alloys.  The mineralogical form of , bunsenite, is very rare.  Other nickel oxides have been claimed, for example: Nickel(III) oxide() and , but they have yet to be proven by X-ray crystallography in bulk. Nickel(III) oxide nanoparticles have recently (2015) been characterized using powder X-ray diffraction and electron microscopy.