Like the histories of the discoveries of many other rare-earths, the tale of erbium reads like a series of mistaken identities. These elements are generally found as oxides and most often together. Chemically, the oxides are very similar and at the time of their first examination were difficult to separate. Thus a sample of "lanthanum" might end up containing two additional elements that no one had bothered to look for. Many chemists thought that the oxides were elements themselves at one time.
The oxide of yttrium (which along with scandium and lanthanum is generally included with the "rare-earths") known as yttria was eventually found to contain erbia and terbia as well, the oxides of, respectively, erbium and terbium. But the two are so similar that they were often confused in early work and what we now call erbium was originally terbium! In both cases, the credit for discovery goes to Mosander (1843 for erbium) and both elements were named for the Swedish town of Ytterby (which, by the way, also lends its name to Ytterbium and Yttrium----certainly some kind of record where naming elements is concerned!).
Like most of the rare-earth metals, erbium is silvery and soft, tarnishing slightly in air.
Stephen R. Marsden (ChemTopics)