iron: the hard, dark metal, and the tools, weapons, and images of unbreakable strength made from it
Iron is the toughest metal the biblical world knew, and the Hebrew word covers the whole life of it: the raw ore dug from the hills, the worked metal bought and traded by weight, the axes and chariots and weapons forged from it, and the picture the metal supplies whenever a writer wants to say something is unbreakable, crushing, or impossible to soften.
The senses below are not different words. They are the same metal seen from different angles, and a single verse can sit on the line between them. The raw material becomes a tool; the tool (the war chariot, the threshing sledge) becomes the very image of an enemy too strong to resist; and that image of hardness is then turned on the sky that will not rain, the neck that will not bend, and the heart whose sin is cut in too deep to erase.
What ties it all together is iron's reputation. Unlike bronze, iron was prized for sheer strength, so when Scripture reaches for iron it is almost always reaching for the idea of force: power that conquers, bondage that cannot be broken, or stubbornness that will not yield.