brilliant silvery-white rare-metal element of boron group in periodic table. The pure metal is very malleable and emits a high-pitched “cry” when bent. It is obtained from zinc minerals but also found in iron, lead, and copper ores. An ingredient in alloys used in fire-door links and sprinkler heads, it is also used to monitor neutrons in nuclear reactors and in manufacture of semiconductor devices. It is valuable for making hermetic seals between glass, metals, and ceramics because it clings to clean glass when molten. It was discovered in 1863 by Ferdinand Reich and Theodor Richter, who named it for the characteristic indigo line in its spectrum.