Iron

Iron was, according to the Tenth Doctor, "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") Its chemical symbol on the periodic table was Fe. (AUDIO: Escape Room)

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)

The planet Megerra was rich in valuable minerals such as iron. (PROSE: Shakedown)

Humans needed a source of iron. (COMIC: The Metal Eaters)

Other references
"Strike while the iron's hot" was a saying which Vivien Rook used as she pressed Lucy Saxon for an interview. (TV: The Sound of Drums)