Minecraft wiki entry for Thorns, associated text: Thorns applies a durability penalty to the armor. It reduces durability by an additional 2 points when inflicting damage. If multiple armor pieces are enchanted with Thorns, the durability penalty is applied to one piece chosen at random, regardless of what level of Thorns that piece has. If the item is also enchanted with Unbreaking, the damage penalty has a chance of being ignored.