Pyronium was a metal which, when refined, possessed a strange and interesting property: it absorbed electromagnetic energy, changing colors in the process through the visible spectrum from black to red, then orange, yellow, green, blue, indigo, violet, and finally to white. It was theorized the atomic structure of the pyronium itself stored the energy; when the quantum shells were unable to accommodate more, the energy was theoretically shunted into a localized hyperspatial matrix, making its absorptive potential extraordinarily high. The metal was so rare it was sometimes referred to as a jewel or gem.
Jax Pavan was known to have a large and unflawed nugget of it, given to him by Anakin Skywalker while he was still a Jedi Padawan.