Gold
Gold was 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)
Nero gave Barbara Wright a gold bracelet. The Animus was able to control her through this. (DW: The Romans/The Web Planet)
Ian Chesterton owned a gold pen. It disappeared when he went onto the surface of Vortis and took it out. (DW: The Web Planet)
Gold was used to power the teleportation devices of the Linktons, twhich allowed them to travel vast, inter-galactic distances. (PHS: The Living Wax)
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. (TV21: 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)
In payment for his service, the Master offered Chang Lee two bags of gold dust that was kept within the Doctor's TARDIS. The Master claimed the dust, along with the TARDIS, had been once his before the Doctor stole it. After the Doctor defeated the Master, he let Lee take the gold dust. (DW: Doctor Who)
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. Adric's badge was gold-edged, allowing for the doctor to grind it into the cyber leader's chest and asphyxiate him. (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)
