Thulium reacts with hydrogen chloride to produce hydrogen gas and thulium chloride. Ytterbium, holmium, thulium, and erbium are other common "dopants" in solid-state lasers. Thulium is often found with minerals containing yttrium and gadolinium. In 1878, erbium ores were found to contain the oxides of holmium and thulium. Independently, Per Teodor Cleve separated it chemically from thulium and erbium in 1879. Other types include beryllium oxide, and calcium sulfate doped with thulium. Its melting and boiling points are also significantly lower than those of thulium and lutetium. Thulium dichloride reacts very vigorously with water. Soluble thulium salts are mildly toxic, but insoluble thulium salts are completely nontoxic. Thulium fiber lasers are being investigated. The isotopes of thulium range from Tm to Tm. Thirty-five isotopes and 26 nuclear isomers of thulium have been detected. With nitric acid it yields thulium nitrate, or Tm(NO). Approximately 50 tonnes per year of thulium oxide are produced. This makes thulium lasers attractive for laser-based surgery.