A quick note about Lanthanum: It isn't radioactive!

But lenses with Lanthanum glass usually are, because trace amounts of Thorium are next to impossible to separate from the Lanthanum. And Thorium is radioactive.

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)!