Silver
From Tardis Wiki, the free Doctor Who reference
Silver is a metal which, in human mythology, could be used to kill a werewolf, particularly in the form of bullets. Silver bullets were also utilised by the Brigadier to defeat the Destroyer. (TV: Battlefield) Similarly, Rosa Caiman had a silver knife to defend herself against werewolves. (AUDIO: Loups-Garoux)
The planet Megerra was rich in valuable minerals such as silver. (PROSE: Shakedown)
When forced to create EDTA to cure arsenic poisoning, Gwen Cooper used a number of ingredients in the antidote, one being silver. (TV: Rendition)
