Adamantium

Adamantium - This is one of the hardest substances in existance. It possesses a largely grey color similar to steel, but has a slight green tint when viewed in the sunlight. It does not exist naturally in the majority of the realms
Adamantium armor
Can be any Medium or heavy armor, but not hide. While you’re wearing it, any critical hit against you becomes a normal hit.
Adamantium weapons
Melee weapons and ammunition made of or coated with adamantium are unusually effective when used to break objects. Whenever an adamantium weapon or piece of ammunition hits an object, the hit is a critical hit.
Cost
The adamantium version of a suit of armor, or a melee weapon or of ten pieces of ammunition costs 500 gp more than the normal version, whether the weapon or ammunition is made of the metal or coated with it.