Mythral

Known as truemetal to the dwarves, this silvery-blue, shining metal is derived from soft, glittering, silvery-black ore found in rare veins and pockets all over, from the depths of the Underdark to surface rocks. Mithral can be combined with steel (varying alloys of iron and carbon) to derive adamantine if one has no access to adamantite ore, but this process is both difficult and known only to a very few dwarves, who do not perform it for nondwarves unless there is a very good reason.
Mithral is the lightest and most supple of metals hard enough to be used in the making of armor; it is extremely valuable.