Azbantium

Azbantium was a mineral four hundred 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 pounding at it (TV: Heaven Sent). However, it was later revealed that this took a period of roughly four and a half billion years, in addition to repeated injury to his hand(s). (TV: Hell Bent)