Gold is quite close to tungsten in density (19320 kg/m^3 for gold vs 19300 kg/m^3 for tungsten). The criminal mind has found use for that factoid: they coat tungsten bars with gold and pass them off all gold (after all, they come close to passing the Archimedes test). However, the speed of sound in tungsten is significantly different than in gold. This fact allows ultrasonic thickness meters† to be used to verify nondestructively that a gold bar is golden thru-and-thru.
