Holmium oxide, also known as holmium trioxide, with the chemical formula Ho2O3, is a compound composed of rare earth element holmium and oxygen, and is one of the strongest known paramagnetic substances along with dysprosium oxide. Holmium oxide is one of the components of erbium oxide minerals. In its natural state, holmium oxide often coexists with trivalent oxides of lanthanide elements and requires specialized methods to separate them. Holmium oxide can be used to prepare special colored glass. The visible absorption spectra of glass and solutions containing holmium oxide exhibit a series of sharp peaks, making them traditionally used as standards for calibration of spectroscopic instruments.