Adamantine, when smelted is ultrahard but it is a costly procedure. Adamantine weapons are useful for damaging the weapons and armour of your opponents. A set of adamantine armour or a shield is nearly impenetrable to normal weapons.
Adamantine is used often by Drow of the Underdark, and their adamantine weapons are usually swords that are magically blessed causing the metal to turn to a jet black color from the unholy magic. Their magical strength and sharpness fade and the metal crumbles to dust if brought into contact with direct sunlight.