Terbium

Terbium is a soft, silvery rare-earth metal discovered by Carl Gustaf Mosander in 1843. It is one of four elements named after Ytterby, a Swedish village whose quarry yielded a remarkable number of new elements; the others are yttrium, erbium and ytterbium. It is scarce and expensive, and comes mainly from clay deposits in southern China.

Terbium compounds glow bright green, which made them the green in fluorescent lamps and tube televisions. Mixed with dysprosium and iron, terbium forms Terfenol-D, an alloy that changes shape in a magnetic field and drives sonar and precision actuators. Like dysprosium, a little terbium can also help a neodymium magnet keep its strength at high temperatures.