The material below is Open Game Content from a 3PP Source
Weapons and armor can be crafted using materials that possess innate special properties. If you make a suit of armor or weapon out of more than one special material, you get the benefit of only the most prevalent material. However, you can build a double weapon with each head made of a different special material.
Each of the special materials described below has a definite game effect. Some creatures have damage reduction making them resistant to all but a special type of damage, such as that dealt by evil-aligned weapons or bludgeoning weapons. Others are vulnerable to weapons of a particular material. Characters may choose to carry several different types of weapons, depending upon the types of creatures they most commonly encounter.