Steelstone
From Will of the Seven Campaign
Steelstone is the strange material used to build the Extraplanar Portal Hub and Lord Aiden's Study. Its appearance is that of a bluish marble stone and metal at the same time. It is cool to the touch.
It is an extremely durable, and flexible material that shares the best qualities of both steel and stone. It bends, but does not stay bent. It does not scratch, crumble, rust.
It is unknown how it is made, and who created it.