Iron

Iron was a chemical element often used by more technologically advanced civilisations for construction.

In human mythology, cold iron (along with salt) could be used to keep supernatural beings at bay. This proved to be true in the case of Azal and Bok. (DW: The Dæmons) The Hervoken were also similarly vulnerable to iron. (NSA: Forever Autumn)

In the 1970s, a meteorite containing sentient iron filings crashed on Earth. The filings were released and began corroding metal wherever they went until they were stopped by the Third Doctor. (TVC: The Metal Eaters)

The Eighth Doctor used iron fillings and gunpowder to create flash-charges. With these he fought off a group of Windigos. (DWM: Bad Blood)