Iron and Steel
It is a known fact that iron has special properties that are lost in the conversion to steel, even though it gains strength and durability.
Iron is a very useful weapon against Fey and Theurges. It has the ability to maintain a small amount of Aether around it, cutting off any Theurgy in the area. As a weapon it can get through the Theurgic defences of the Fey and as an armour it is useful to protect yourself from Theurgy.
Many Theurges, myself included, wear specially stitched gloves on their off-hand that are laced with iron. While not a comfortable material to wear and often worn over silk, it can allow the Theurge to re-stitch the Aether by moving the hand through it. This cancels any Theurgy in progress and can also be used in Duels as a defence, though in Official IGT Duels this material is banned as it ruins the sport.