Iron is the alchemists' Mars. The symbol is a circle with an arrow leaving it at the upper right, and it is still in use in biology as the sign for male. The associations are consistent across the tradition: war, blood, heat, the colour red, the sharpness of a blade. Rust is red, arterial blood is red, and the planet Mars is visibly red in the sky. The language kept the link long after the theory went: martial, martial law, and the iron in the blood, which is literal, since haemoglobin carries oxygen on an iron atom.
Before smelting there was meteoric iron. Iron arriving as a meteorite is already metallic and can be cold-worked, and objects made from it appear in archaeological contexts long before any furnace could reach the temperatures needed to reduce iron ore. Ancient Egyptian texts use a term conventionally translated as iron of the sky, and the iron dagger buried with Tutankhamun has been shown by X-ray fluorescence analysis to be meteoritic, matching the nickel and cobalt proportions characteristic of meteorites.
That is a genuinely important fact for a book of correspondences and it needs no embellishment. For a considerable period the only iron available anywhere was iron that had fallen from the heavens, and it was rarer and more valuable than gold. The association of the metal with the sky is older than its association with the planet Mars, and it rests on straightforward observation rather than on symbolism.
Smelted iron appears in the Near East during the second millennium BCE and becomes general after roughly 1200 BCE. Iron ore is far commoner than copper or tin, so once the technique existed, iron tools and weapons could be made in quantities that bronze never permitted. That is what the phrase Iron Age is describing.