Illium
From superman.nu: Supermanica
Illium
Illium is a rare metal used by the Interplanetary Circus to keep caged the Wild Man from the planet Zaga, as it is the only substance it cannot devour (S No. 145, May 1961: "The Interplanetary Circus").
Illium is also used to make the statuette for Krypton's coveted Science Prize (S No. 173/2, Nov 1964: "Tales of Green Kryptonite No. 1").
