Sky Steel Rock, known as the least valuable, yet most valuable stone. This kind of stone had natural resistance towards mana, and it was said that elements couldn't pass through it easily, making it completely immune to spells.
That thing was extremely hard, and since it was immune to the power of elements, no swordsman under the Heaven Rank could dream of destroying that thing. Only those with enough pure destructive power could destroy Sky Steel Rock.
It was very rare, only gatherable in small quantities, and difficult to extract. It was the best material for constructing city walls or forts, and there wasn't much of it in Noscent. During the 3rd Dynasty, only the city walls of the imperial city used bricks made of Sky Steel Rock.
Such a large amount of Sky Steel Rock must have been mined by Bill George's puppet army.
