Iron is the material composing a large portion of the items in the Dungeons of Doom. Iron can corrode or rust. Rubbing iron on a touchstone produces its signature "scritch, scritch." Hitting a brown or black pudding with iron splits it into two. Polypiling iron objects results in an iron golem.
Wearing any armor made out of iron will inhibit casting of spells, so they're generally not used among the wizard class.