Orichalcum

Orichalcum (Sometimes Known as Seki Seki) is a white, marble-like stone located in the deeper regions of the underground, as well as within floating islands. it is a nigh invincible material, that cannot be mined or moulded by magic. Any magic used on Orichalcum is instantly absorbed by it. There are known instances of lattices, similar to a tree's root structure, of Orichalcum preventing magical travel-or in more dire cases, the use of magic altogether- nearby due to its powerful antimagic Aura.
Orichalcum is believed to be the petrified remains of The World Tree's Roots and branches, suspended out of time. There are myths that The Gods are able to affect Orichalcum, making legendary artifacts from it. There is no evidence to prove this, however.