Europium

Europium's half-filled 4f⁷ shell makes it unusually happy in the 2+ state, which is why it behaves chemically unlike its neighbours and why geochemists prize the "europium anomaly" — the depletion or enrichment of europium relative to the other rare earths in a rock records whether plagioclase feldspar crystallised out of the magma, and the same signature in Apollo samples helped establish that the Moon once had a magma ocean. Practically, europium is a phosphor: Eu³⁺ gives a saturated red, Eu²⁺ a blue, and the arrival of a bright, efficient red in the 1960s is what finally made colour CRT television commercially viable. Euro banknotes carry europium compounds that fluoresce under UV as an anti-counterfeiting mark.