Element X is a metaphysical substance that forms the basis of the New Gods' technology. Given it's metaphysical nature, it is most likely the greatest fuel known in the DCU. Invented on Apokolips separately by Himon and Darkseid's technicians, it is the energy source for such Fourth World devices like Mother Boxes and Metron's Mobius Chair (which gave him quite a bit of trouble until he learned to harness the element properly). It is also used as a matter threshold for Boom Tubes.
In Final Crisis 7, Superman harvests the X Element from Metron's Mobius chair in order to power his miracle machine.
In Rebirth during Dark Nights: Metal, it is revealed that element X; also called the tenth metal, related to the eight and ninth metals. Is the substance comprising of all DC continuity; that being the positive Multiverse, the Dark Multiverse, the Antimatter Universe and beyond even the depths of the Source Wall itself.
Having been dubbed as the metal of pure possibility stemming from the Forge of Worlds wherein entire universes are crafted and made. Its said that whosoever wields the Tenth Metal will have his/her first words etched into the hall of kings. For with it, those who have it in their possession can bring their imaginations to life through its use. Being able to take whatever form the user wants and arm whoever they wish to bequeath it too.
But it loses it's effect the deeper into the Darkness one goes. Because it is the metal from which all things spring into existence from. Every single thing within reality is comprised of trace amounts of Tenth Metal, it is literally the stuff of creation. Used in tandem with copious amounts to interconnect with every one and every thing, it can rewrite the nature of all Reality.