Doonium

Following the end of the Clone Wars, the Empire extensively mined and stockpiled large amounts of doonium, secretly utilizing it in the construction of the Death Star superweapon, for which, along with dolovite, it was essential in shielding the hypermatter reactor core and focusing dish. Around this time, three people in particular, Thrawn, Eli Vanto and Nevil Cygni all noticed that doonium was being transferred to an unknown point in the galaxy, though only Cygni, the rebel agent "Nightswan," actively fought to prevent the Empire's shipments of the mineral in fear of whatever the Empire was creating. Thrawn later learned from unguarded dispatches the true nature of the doonium's secret use.