He is best known for his discovery of the chemical elements holmium and thulium. In 1879 Cleve discovered holmium and thulium. Humans typically consume about a milligram of holmium a year. In 1878, erbium ores were found to contain the oxides of holmium and thulium. Dysprosium titanate, like holmium titanate and holmium stannate, is a spin ice material. Natural holmium contains one stable isotope, holmium-165. Holmium and thulium are used in very high power movie lighting models. Holmium oxide is used in making specialty colored glasses. The most efficient separation routine for holmium oxide from the rare-earths is ion exchange. Large amounts of holmium salts can cause severe damage if inhaled, consumed orally, or injected. The biological effects of holmium over a long period of time are not known. Ytterbium, holmium, thulium, and erbium are other common "dopants" in solid-state lasers. Holmia was later found to be the holmium oxide, and thulia was thulium oxide. Elemental holmium is a relatively soft and malleable silvery-white metal. Holmium is quite electropositive and is generally trivalent.