Gold

From TARDIS Index File, the free Doctor Who reference.

Gold is a chemical element much prized by Humans, and a major weakness of Cybermen.

The Animus could control beings that were in contact with gold, which led to it controlling the Zarbi. (DW: The Web Planet) The Keratin could similarly use gold to conduct and amplify their psychic powers. (SJAA: The Glittering Storm)

Lady Peinforte dipped her gold-headed arrows in poison as her "calling card". (DW: Silver Nemesis)

Flidor was rich in blue-veined gold (DWA: The Dalek Book), which was used to construct the Dalek Emperor's casing. (DC: Genesis of Evil)

Prospecters flocked to Mal Oreille, mistakenly believing its yellow-algae infested seas to be rich in gold deposits. (BFBS: Missing Adventures: Biology Lesson on Mal Oreille)

Owing to its non-corrosive nature, gold essentially choked the Cybermen's respiratory systems. The glittergun, a weapon used during the Cyber-Wars, fired gold dust at its targets to exploit this weakness. (DW: Earthshock) Gold appeared to affect some varieties of Cybermen in the way that silver affected werewolves, so that gold coins or gold-tipped arrows fired at them had the same effect. (DW: Silver Nemesis). Gold affected the Cybermen of Pete's World before extensive research eliminated the weakness.

During a time period in which the Cybermen had been reduced to small remnant groups wandering around the galaxy, one group tried to take revenge by making a desperate attempt to blow up the remnants of the planet Voga, a planetoid of pure gold that had wandered into the Sol system and become a moon of Jupiter. They hoped that this would disrupt their enemy's supply of the element. (DW: Revenge of the Cybermen)