Quote Originally Posted by Yanagi View Post
I've never actually seen a clean explanation for why iron is a ward against evil stuff, and the story exists in multiple folklore systems, not just Celtic tales of faeries. For example, in the Caucus Mountains and in Tibet.
Iron (with "cold iron" being a poetic term for iron weapons, like we use "cold steel" today) is generally seen as a sign of human industry and ingenuity, since we take it out of the earth and do all sorts of cool and crazy stuff with it, so it is philosophically opposed to supernatural creatures associated with the wilderness, nature, and the "old order" in general. When people started to put iron horseshoes above their door, iron fences around graveyards, etc., it started to take on the general "protection against supernatural stuff" associations; that part may also have been influenced by natural magnetic properties of meteoric iron, as you noted, but I'm not aware of any writings on the subject.

Silver is attributing healing and cleansing properties by some New Age systems, but its unclear in the past if it had a firm mystical power set. The idea of silver as weapon versus supernatural things is comparatively modern and seems to come from a French story.
The tradition is definitely much older than New Age stuff. Silver has antimicrobial properties and has been used in treating wounds and diseases at least as far back as Hippocrates. This connection with health and purification is seen in myth, with ancient Egyptians making protective charms and amulets out of silver, Dian Cecht (Celtic god of healing) supposedly giving a heroic soldier a hand of silver after his own was lost, and so forth, and the creatures it is supposed to protect against are generally associated with disease or corruption (lycanthropy is transmitted from person to person, demons corrupt you, vampires do both, etc.).

Silver-as-weapon-against-evil-things isn't particularly modern, either; the story you're thinking of, the killing of the Beast of Gévaudan, is what specifically sparked the idea of silver bullets being effective against werewolves, not usage of silver in general. Silver taking on a close association with werewolves in particular since the Middle Ages is likely due to silver having an alchemical association with the moon, so werewolves transforming by moonlight combined with the prevalence of sympathetic magic in various believe systems would imply that silver has some sort of special power over them.