Quote Originally Posted by Ole
Lanthanum belongs to a group of elements called "rare earth elements" for historic rason only. They are not particularly rare, nor particularly earthy, but the name seems to have stuck. Most of them are named for Ytterby, a village just outside Stockholm, Sweden (Ytterbium, Yttrium, Terbium, Erbium, Holmium, as well as Scandium)!
Rare earths are generally found as oxides. As I have heard, "Rare" refers to the fact that it is very hard to separate the metal from the oxygen.

Rare earths are very heavy atoms. Silicon and oxygen (the main components in glass) are rather light. I suspect that the rare earths increase the refractive index of the glass because of this. I should probably google this before saying something stupid...