In that case the emitted light has exactly the same properties as the light used to trigger it. This makes a system containing a lot of excited atoms in effect an amplifier for light, an insight that has led to the development of lasers. The element erbium Only 91 different atoms occur naturally, and together with a handful of artificially produced atoms they are known as the elements. Most of our every-day environment is build from just about 20 of those, such as carbon, oxygen, nitrogen, hydrogen, iron, aluminum and silicon. Other elements are encountered much less often, but still can have practical use in some specialized applications.
Erbium is one such element, that belongs to a group known as the 'rare earth metals'.
Chemically all elements from this group are behaving pretty much the same. The reason is that the differences are hidden fairly deep inside the atom. Electrons in very elliptical orbits are much more important for how the atom interacts with its surroundings, and thus determine the properties of an element. Carbon is a solid because its four outer electrons make the atoms cling together. Neon is a gas because with eight outer electrons it is already overcrowded, and does not like other atoms in its neighborhood.