Iron's pairing with Mars is one of the most consistently attested and intuitively motivated correspondences in the entire planetary metal scheme, and its symbol, a circle topped by an arrow pointing up and to the right, is identical to the astrological glyph for Mars and to the biological symbol conventionally used today for the male sex, a shared glyph whose ultimate origin, whether the symbol first meant the planet, the metal, or Mars the god of war directly, is treated by historians of symbolism as genuinely difficult to disentangle given how early and how thoroughly the three associations, planet, metal and god, had already merged by the time the alchemical glyph tradition was first written down.
What is not in doubt is the correspondence's practical grounding: iron's dominance as the metal of weapons and armor gave writers across the classical, Islamic and European alchemical traditions an obvious, shared reason to link it with the god and planet of war, a correspondence that required no elaborate theoretical justification to feel immediately intuitive. Iron's practical dominance also meant it was, of all seven classical metals, the one whose supply and working skill most directly determined a society's military and agricultural capacity, giving the Mars correspondence a weight far beyond mere symbolism.