Dark star alloy

Dark star alloy was a material that "goes through armour-plating like a knife through people." In the olden days on Gallifrey, the Doctor gave the Master a brooch made out of dark star alloy. It could even pierce a Dalek's casing, something the Dalek professed was immune to anything only to realise it allowed aged and vengeful Daleks to enter, who then mercilessly destroyed it. (TV: The Witch's Familiar)