Erbium, a metal supposed by Mosander to exist, together with terbium and yttrium, in the mineral gadolinite of Ytterby in Sweden. Mosander in 1843, while examining the crude yttria obtained from gadolinite according to the method of Berzelius, came to the conclusion that there was a mixture of three earths of different basic power; these he named, in the order of their bases, erbia, terbia, and yttria. Subsequent investigations of different chemists have thrown considerable doubt on the existence of terbium. Bahr and Bunsen separate erbia and yttria by precipitating with oxalic acid, treating with potassium sulphate, converting into nitrates, dissolving, and repeatedly crystallizing; nitrate of erbium is ultimately obtained, containing no appreciable quantity of yttrium. Erbium oxide, prepared by ignition of the nitrate or oxalate, is a faint rose-red powder, and glows with an intense green light at high temperatures, which in the spectroscope exhibits a continuous spectrum intersected by a number of bright bands. Solutions of erbium salts give an absorption spectrum exhibiting dark bands. The salts of erbium have a rose-red color, and closely resemble in taste and reactions the compounds of yttrium.

The atomic weight of erbium is 112.6. The metal has never been isolated.