Dark star alloy

Dark star alloy was a material that went "through armour-plating like a knife through people." On Gallifrey, the Doctor gave the Master a brooch made out of dark star alloy. used the brooch to pierce a Dalek's casing, allowing the Daleks left on Skaro to rot and liquefy to pass through the armour and kill the creature within. (TV: The Witch's Familiar)