Kiirium was an armor material used to shield starship engines and heavy weapons. By the end of the Galactic Republic, it was considered antiquated but serviceable. It was transported in easily smelted ingots.
However, during the time of Xim the Despot, it was a valuable war material. He recognized the need to obtain vast quantities of the metal for his growing war machine, and loaded the treasure ship Queen of Ranroon with kiirium, mytag crystals, and beam-tubes to hoard it in his vaults on Dellalt.
Millennia later, Han Solo discovered the fabled treasure of Xim consisted of this stockpile of war supplies, now worthless due to advances in technology.