-In- is a chemical element with atomic number 49. This rare, very soft, malleable and easily fusible post-transition metal is chemically similar to gallium and thallium, and shows the intermediate properties between these two. Indium was discovered in 1863 and named for the indigo blue line in its spectrum. This blue line was the first indication of its existence in zinc ores, identifying indium as a new and unknown element. The metal was first isolated in 1864. Zinc ores continue to be the primary source of indium, where it is found in compound form. Very rarely the element can be found as grains of native (free) metal, but these are not of commercial importance.