Silver

Silver is a metal that can take in spirit energy. The spirit fortifies the metal making it stronger to the point of it being unbreakable in the right hands. It is through the use of said metal that armors can be made lighter, and weapons thinner.
The only better material to make weapons is volcanic glass, known as obsidian. The only reason is obsidian takes in and holds all spirit energy that goes inside it. Unlike silver which is only powered by the person holding the weapon.