Dikironium is an element which, in the mid-23rd century, was not believed to exist in nature until the crew of the USS Enterprise encountered a cloud creature that was composed of that element. (TOS: "Obsession" ) According to the Star Fleet Medical Reference Manual, (p. 17) dikironium, symbol Dk, had the atomic weight 300 and atomic number 112. It was first discovered on the planet Vulcan.