Beryllium

Beryllium is the metal in emerald and aquamarine, both of which are the mineral beryl coloured by trace impurities. Pound for pound it is stiffer than steel, and because its low atomic number makes it nearly transparent to X-rays it is the standard window material on X-ray tubes and detectors; the James Webb Space Telescope's 18 mirror segments are gold-coated beryllium chosen for dimensional stability at 40 K. Its place in physics is secured by a 1932 experiment: James Chadwick fired alpha particles at beryllium foil, measured the uncharged radiation that came off, and identified the neutron. The catch is toxicity. Inhaled beryllium dust causes chronic berylliosis, an irreversible granulomatous lung disease, and exposure limits are among the strictest of any industrial metal.