A way to do the "cold iron" trope

I’ve got an idea of how to pull of the trope of "cold iron" in a simple, yet effective and flavorful way.

First of all, "cold iron" in this context would be a purely poetic way to refer to *any* kind of iron (including steel), as in the original real-world folklore, with the Fae’s weakness to it being the result of the inherent magical symbolism of "wild nature being conquered by human civilization and progress".

Second of all, iron actually does *not* just deal increased damage to Fae; in fact, it inflicts the same amount of harm it would do to a regular human (because *anyone* will dies from being shanked, no matter the material). It´s just that it’s *everything else* that deals *less* damage. Due to supernatural biology and/or access to healing magic, Fae are able to shrug off non-magical physical damage relatively easily, and in order to counter that, weapons made from any non-iron material need to be enchanted separately first. Iron’s special because it’s the only material that *doesn’t* need magical enchantement in order to harm Fae normally; enchanting would still make it even more deadly, but any old kitchen knife or farming/gardening tool will still do the job in a pinch.

It would also explain why nailing an iron horseshoe above your front door keeps the Fae from entering: it’s not a magical ward or anything (like how a crucifix repels vampires and demons), but instead a warning sign that lets the Fae know that this is a household with relatively easy access to iron objects and that the residents will thus not be defenseless against them.

Discuss and share your own explanation for cold iron.