Beryllium, a relatively rare, light element, is formed by collisions of carbon and oxygen atoms in the space between stars. Sometimes, this interstellar billiard action will break up atomic nuclei into the lighter elements lithium, beryllium, and boron. On Earth, beryllium is found inside emeralds; it's produced for industrial use through chemical reactions. The JWST team chose beryllium primarily because it remains stable even at incredibly cold temperatures. The mirrors will be coated with gold to increase their infrared capability.