Iron is the metal of Mars in the classical planet-metal scheme, and the pairing is among the most fitting: iron is the metal of weapons and war, hard, strong and blood-red when it rusts, and Mars is the god of war, so the alchemists gave iron and Mars the same symbol, the circle with an arrow, the same sign that now stands for the male. Iron's arrival, later than copper and bronze, transformed warfare and agriculture alike, and its strength made it the practical metal of tools and blades.
Ritually, iron carries a strong and widespread body of protective folklore: cold iron was held across Europe to repel fairies, witches and the evil eye, the iron horseshoe nailed over the door being the best-known survival, and iron nails and blades were used in countless apotropaic customs. Some traditions, conversely, barred iron from sacred precincts, holding it too worldly or violent for the altar. Its Martian associations are of force, courage, aggression and the will, the assertive and combative principle among the metals.
Sources The Secrets of AlchemyLawrence M. Principe with An Encyclopedia of Fairies: Hobgoblins, Brownies, Bogies, and Other Supernatural CreaturesKatharine M. Briggs