Azbantium

Azbantium was a mineral 400 times harder than diamond.

A twenty feet thick barrier of it appeared within the Twelfth Doctor's confession dial. In an effort to escape his confinement, the Doctor wore it away by punching at it (TV: Heaven Sent) over a period of four and a half billion years. (TV: Hell Bent)

The spade in the Twelfth Doctor's confession dial was made of an amalgam of duralinium and dwarf star alloy, a combination theoretically strong enough to cut azbantium. (PROSE: The Veil)