Iron

According to the Tenth Doctor, iron was "a malleable ductile ferromagnetic metallic element, found mainly in haematite and magnetite." (PROSE: Forever Autumn) The First Doctor held that it had a melting point of 1,539 degrees centigrade. (TV: "The Unwilling Warriors")

In human mythology, iron was an "old magical defence". Because the animated gargoyle Bok believed in magic, the Third Doctor kept Bok at bay with an iron trowel and an old Venusian lullaby. (TV: The Dæmons) The Hervoken were also similarly vulnerable to iron. (PROSE: Forever Autumn) Mithras used a sword of pure iron to kill a Nimon scout. (AUDIO: Seasons of Fear)

The Tenth Doctor tasted iron after he licked a portable television. (TV: The Idiot's Lantern)

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. (COMIC: The Metal Eaters)

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

Iron could rust. (AUDIO: The Macros)