The element iron has a special status: it carries oxygen on our blood; it’s the most abundant element in the earth’s crust; and it has the most stable atomic nuclei. More to the point for fantasy gaming: "cold iron" is said to ward off or harm fairies, ghosts, and/or witches.
In the novel Enterprise of Death by Jesse Bullington, magical attitude is inversely related to iron in the blood. A necromancer explains it this way:
“Iron, as I’ve told you, is one of the only symbols that represents what it truly is, here and on the so-called Platonic level of reality...Because it is true material and not just a symbol of something else, iron restricts our ability to alter the world, be it talking to spirits or commanding symbols or however you put it.”
Not only does this nicely tie some of the real properties of iron with its folklore properties, but it would have some interesting implications in fantasy games. Prohibitions against metal armor and the working of magic make sense in this light. Even more interestingly, it might it explain why D&D mages tend to be physically sort of weak--they need to be somewhat less robust in order to work magic well. Maybe higher Constitution scores actually impairs magic, or impairs the “level” a mage can advance too? That might also example the traditional dwarven poor magic aptitude: they’re hardy, creatures of the earth (where iron’s abundant).