Mithrilium was the hardest known substance in the Teggedon Spiral, several thousand times stronger than diamond. Mithrilium was a silvery-gray alloy of several elements including annunakium, silver, and titanium. One of the most expensive materials to manufacture, mithrilium had very little practical applications. The main entrance to the Xaronese Imperial General Headquarters was protected by doors of solid mithrilium which could withstand small atomic explosions. The metal adamantium was created to be a cheaper substitute for mithrilium.