SYSTEM REFERENCE DOCUMENT 3.5
In addition to magic items created with spells, some substances have 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 based on their creature type or core concept. Some are 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 campaign and types of creatures they most commonly encounter.