Mortals believe that iron is able to prevent faeries from controlling your mind. It is for this reason that in the village where the Archeron family lives, due to its proximity to the Wall that divides the Mortal Lands from the faerie kingdom of Prythian, most of the inhabitants wear some kind of iron garment, be it rings, bracelets, necklaces, etc.
Nesta and Elain Archeron both had iron bracelets, which were bought by Nesta, who apparently didn't bother to buy one for her other sister, Feyre Archeron. Also on one occasion Feyre sees a woman with a full necklace of braided iron around her throat, according to her, the woman had money since buying an iron necklace of that size would cost a lot.
When Feyre is taken by Tamlin, the High Lord of the Spring Court, to his lands, his emissary, Lucien Vanserra, explains that iron does not actually have an effect on faeries, which is just a lie they told mortals years ago, as well as the belief that faeries can't lie. The only thing that is true is that ash wood can hurt and even kill them.
- Although Throne of Glass, another series written by Sarah J. Maas, is set in the same universe as the A Court of Thorns and Roses series, in it iron is the only material that can repel and contain magic, while in A Court of Thorns and Roses iron does nothing to faeries, despite what mortals believe.