Halkonite steel

Halkonite steel was a metal alloy of extraterrestrial origin. Despite being light, once the metal was forged, its molecular structure would become permanently and perfectly stable and it became flawlessly indestructible. This metal could not be affected by either material shifts or matter state changes, meaning it was immune to changes such as rust, decay, and corrosion. According to Sarah Jane Smith, it was completely impenetrable. Nothing could get through it, not even thoughts.

In the 2000s, Sarah Jane Smith used a box made of Halkonite steel to contain a fragment of the Weserbergland meteorite where the Pied Piper had been trapped. (TV: The Day of the Clown)

Behind the scenes
According to the official SJA website Halkonite steel is from a place called Halkon.