Adamantine

This alloy, of five-eighths adamant to two-eighths silver and one-eighth electrum (itself a natural alloy of silver and gold) retains the hardness of adamant, but combines it with a rugged durability that makes adamantine so hard to shatter that it is the favored substance for the making of war hammer heads, the best non-mithril armor, and harbor chains. (By one of the miracles granted by the gods, adamantine can also be derived by combining steel and mithril if one knows how. Adamantine is black, but has a clear green sheen in candlelight a sheen that sharpens to purple-white under the light given off by most magical radiances and by will-o-wisps.
Adamantine is tricky to make, and must be forged and worked at very high temperatures by smiths who know exactly what they are doing and who have access to special oils to slake and temper the hot metal in. Almost all such expert smiths are dwarves, as the Deep Folk guard the secrets of working adamant jealously, but a priest or wizard seeking to enchant items can make use of finished adamantine items and need not necessarily have to work with a smith to create an adamantine work anew.
• Adamantine weapons deal double damage to objects and structures. This doesn't allow a weapon to deal damage to something it otherwise couldn't, such as a bludgeoning weapon dealing damage to a hanging rope.
• When a critical hit is made against a creature wearing adamantine armor, it becomes a normal hit.