Europium

Europium is the most reactive of the rare earths: a fresh piece tarnishes quickly in air and reacts readily with water. Eugène-Anatole Demarçay isolated it in 1901 and named it after Europe. It is mined with other rare earths, mostly in China.

Its talent is light. Added in tiny amounts to a crystal, europium makes it glow a pure color when struck by electrons or ultraviolet light. Europium-doped yttrium compounds gave old tube televisions and fluorescent lamps their red, and europium phosphors still help some white LEDs produce richer reds and greens. Because the glow is hard to fake, europium compounds are also used in security marks on banknotes that light up under UV lamps.