Halkonite steel

Halkonite steel was a metal of extraterrestrial origin. According to Sarah Jane Smith, nothing could get through it, not even thoughts. In 2008, 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.