This article contains spoilers for SWORD #6 by Al Ewing and Valerio Schiti.
The X-Men's new metal is better than adamantium or vibranium. The Jonathan Hickman era has seen the entire mutant race gather on the living island of Krakoa, where they're learning to synergize their powers in order to achieve remarkable feats. They've learned how to conquer death itself, and are resurrecting the millions of mutants who've been killed over the years.
But Al Ewing's SWORD series has seen them go one step further. Having defeated death, the X-Men are now plundering the heart of creation, entering a dimension described as the "mysterium tremendum," the "elemental cosmic heart," the "white-hot room of secret fire," and the "excelso prae omnibus aliis." The third of those mysterious statements is the only one that should resonate with any X-Men reader, because the White Hot Room is a realm that lies at the heart of the Phoenix Force. They're retrieving something from this place, and in SWORD #6 they reveal just what they've found - a new metal.
The mutants have named this new metal, "mysterium," and according to their scientists, it is more precious and valuable than either adamantium or vibranium. Newly-acquired mysterium is apparently easy to shape, but once it has "set," it has strength on par with adamantium. It conducts electricity better than graphene, and barely conducts or retains heat, making it perfect for producing spaceships with hulls a tenth of normal thickness that can literally fly through a sun. The galactic powers immediately recognize the significance of mysterium, using it as the basis for a new currency called "The Sol."
Mutants have long played a major role in galactic politics, with Professor Xavier becoming the consort of Shi'ar Empress Lilandra; his daughter Xandra is currently ruler of the Shi'ar. But their position is changing now because they don't want to be considered random interlopers anymore. The Hellfire Gala has served as the X-Men's pitch to become a galactic power in their own right, with the mutants terraforming Mars as the new self-declared capital of the Solar System and Storm appointed as Regent of Sol. Most of the other galactic powers see enough value in mysterium to accept this, particularly given many of them are currently suffering economic crises in the aftermath of Knull's rampage. Spartax and the Galactic Rim Collective in particular immediately embrace the Sol as part of their recovery.
The mutants' new metal is as tough as adamantium and of infinitely more valuable than vibranium, immediately becoming the foundation of the entire galaxy's economy. Marvel claimed this phase of the X-Men books should be known as the "Reign of X," but nobody imagined the sheer scale of it all.