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. (DW: Battlefield)
The planet Megerra was rich in valuable minerals such as silver. (NA: Shakedown)
When forced to create EDTA to cure Arsenic poisoning, Gwen Cooper used a number of ingredients in the antidote, one being silver. (TW: Rendition (TV story))
