An element of the chromium group, found in certain rare
   minerals, as pitchblende, uranite, etc., and reduced as a heavy, hard,
   nickel-white metal which is quite permanent. Its yellow oxide is used
   to impart to glass a delicate greenish-yellow tint which is accompanied
   by a strong fluorescence, and its black oxide is used as a pigment in
   porcelain painting. Symbol U. Atomic weight 239.