The Iron Skin Gem is an essence gem which causes players to take no damage from traps, and also improves their Body attribute. It is a stronger version of the Thick Skin Gem. It can be found in the Lotus Assassin Fortress, in jar within the Upper Mine.
In-game description: "The power of this gem toughens the skin until it as strong as iron. Traps and other undirected attacks will have no effect at all upon the wielded. Calculated, cunning strikes from an intelligent opponent will still harm the user, but the strengthened skin will help absorb damage."