Iron

Iron is a chemical element often used by more technologically advanced civilizations for construction.

In Human mythology, cold iron (along with table 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 Doctor. (TVC: The Metal Eaters)