Terbium

Terfenol-D — terbium, dysprosium and iron, named for the Naval Ordnance Laboratory that developed it — is the strongest magnetostrictive material known, stretching about 0.2 % when magnetised. That is enough to build low-frequency sonar projectors, precision actuators and the flat panel speakers that turn a window into a loudspeaker. Terbium is also the green in the three-phosphor blend that made compact fluorescent lamps possible, working alongside europium's red and blue. It is one of four elements named after the same Swedish village, Ytterby, whose feldspar quarry yielded yttrium, terbium, erbium and ytterbium — the densest concentration of element names in one place on Earth.