Indium is a post-transition metallic element, rare in the Earth's crust. The metal is very soft, malleable and easily fusible, with a melting point higher than sodium, but lower than lithium or tin. Given its physical nature and position in the periodic table, it counts as a heavy metal, but in common circumstances is not a toxic hazard. Chemically, indium is similar to gallium and thallium.
The indigo blue line in the spectrum of indium was observed in 1863. That line was the first indication of an unknown element in zinc ores, and when the free metal was isolated in the following year it was named indium after the color of the light that had provided a clue to its presence.
Currently the demand for indium is driven by the manufacture of transparent electrodes from indium tin oxide (ITO). The electrodes are used in liquid crystal displays and touchscreens. The metal also is used in a wide range of alloys; one of its first large-volume applications was in high-performance bearing alloys for aircraft in WWII. It is also used for making particularly low melting point alloys, and is a component in some solders. One of its unusual attributes is that, like gallium molten indium wets glass, so that it can be used as a solder in glass seals. It also is used in a wide range of electric and electronic roles, and has been used in superconducting alloys.