Iron: the anti-magic

Iron is the core of the plane of Flesh. As such, it is antithetical to both spirits and magic.

Magic cannot affect objects made of iron. No spell can be cast on an iron nugget, an iron statue, an iron sword, or an iron shield—the spell fizzles, as if countered.

You cannot cast a spell if you have a significantly-sized piece of iron or steel anywhere on your person.

What is a significant size of iron? A coin is okay. A cast iron pan is not. A steel sword is not. Metallic armor is definitely not.

If someone is wearing iron armor that covers at least 75% of their body, they cannot be targeted by any spells. (Magic might still have an effect on them though—a wall of fire conjured by a spell still burns an iron-clad knight, but the knight herself cannot be targeted with a spell to light her clothing with heavenfire.)