Iron

Iron is known to have an effect on Faeries or Mages in general- it can dampen their connection to magic and is painful for those dealing with the effects. Little is known about why this is the case, but it has often been used in the wars between human and faery- since all faery have at least a little connection with magic. Scholars have hypothesized that humans have merely become accustomed to the presence of iron, but can be as impacted by it's presence as faeries if they have stronger ties with magic. However, since Mages are extremely rare among humans in modern times, there is not a way to test it.

There are old myths that iron didn't used to be as problematic- and some scholars hypothesize that it may be tied to the Fall of high magic. Very little information about magic from those times survives, however, so nothing can be verified.