In 1843, Carl Gustaf Mosander discovered terbium and erbium as components of yttria.
However, this discovery was hotly contested.  Spectroscopist Nils Johan Berlin denied that the two elements existed, failing to confirm the existence of "erbia" and suggesting that its name be applied to "terbia".  
In 1864, Marc Delafontaine used optical spectroscopy to conclusively prove that yttrium, terbium, and erbium were separate elements. Ironically, however, the confusion that had been introduced between the names continued. Mosander's proposed names were switched, giving the amethyst compound the name "erbium" oxide and the yellow substance the name "terbium" oxide, instead of the other way around as originally proposed.