Germanium

Mendeleev's boldest gap was eka-silicon, predicted in 1871 with an atomic weight near 72 and a density near 5.5. Clemens Winkler found germanium in the mineral argyrodite in 1886 and measured 72.6 and 5.35, and the match was close enough that the remaining sceptics gave up. The first working transistor, built by Bardeen and Brattain at Bell Labs just before Christmas 1947, was a germanium point-contact device, and germanium ran the early transistor industry. Silicon took over in the late 1950s not because it was a better semiconductor — germanium's carrier mobility is higher — but because silicon's oxide is a stable insulator and germanium's is water-soluble. Germanium came back through the side door: GeO₂ raises the refractive index of fibre-optic cores, and germanium lenses are transparent in the infrared, so most thermal cameras look through it.