Holmium(III) oxide, or holmium oxide is a chemical compound of a rare-earth element holmium and oxygen with the formula Ho₂O₃. Together with dysprosium(III) oxide (Dy₂O₃) holmium oxide is one of the most powerfully paramagnetic substances known. The oxide, also called holmia, occurs as a component of the related erbium oxide mineral called erbia. Typically the oxides of the trivalent lanthanides coexist in nature and separation of these components requires specialized methods. Holmium oxide is used in making specialty colored glasses. Glass containing holmium oxide and holmium oxide solutions have a series of sharp optical absorption peaks in the visible spectral range. They are therefore traditionally used as a convenient calibration standard for optical spectrophotometers.