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Thundrainum is an alloy that is extremeley lethal to all Thunderians.
It is also called "fire rock" on Third Earth. It was considered dangerous, so the denizens of Third Earth stopped using it. The element has been used heavily by the enemies of the ThunderCats; it can be forged into weapons, turned into ammunition and, once, cages were crafted from it. There is a mountain on Third Earth that has substantial Thundrainium deposits called Fire Rock Mountain. The only know way for a ThunderCat to be unaffected by Thundrainium is to posses the power of the Star of Thundera, which when summoned is transformed into a locket. Mumm-Ra once opened the locket, which in turn destroyed Fire Rock Mountain.
Grune is the only Thunderian shown to be immune to the effects without the Star, when he wielded a Thundrainium mace in battle against Jaga as a ghost. How and why this happened has never been established.