Tromium

Tromium is a crystalline mineral which has the effect of placing anything encased within it in suspended animation. While comparatively fragile, it is exceptionally resistant to changes wrought by the passage of time.

It is presumably named after Element Lad's decimated home planet, Trom.

It comes in two forms:

• Green Tromium is the purest form, used by Element Lad to protect his teammates from the effects of the Rift, after half the Legion Outpost was swallowed by it. While returning the Outpost to the material universe, he was then thrown back in time, and would also transform himself into the same mineral, albeit sometimes maintaining an outer, apparently organic, skin, enabling him to survive the "billions" of years before catching up with the Legionnaires' timeline.
• Silver Tromium is an alternative form, apparently created by the effect of resonant electricity being stored in Green Tromium for some time. The reconstituted Live Wire's Element Lad-like body was composed of it, and it was the only substance he was able to create using Element Lad's power. A thin layer of it, generated by Live Wire, protected Kid Quantum (Jazmin Cullen), LeViathan and Superboy/Kon-El from the stress of time-travel without a ship in a Darkseid-spawned temporal crisis, while a thicker layer placed Singularity in stasis moments before he would have explosively overloaded his powers in an attempt to kill the Legion.