Adamantium, while TECHNICALLY an unobtainium-like concept, has always been considered an indestructible metal, even when it was known in mythology as Adamant. Sometimes this was referring to diamond, sometimes it literally meant a metal that would not break. Tolkien may have created mithril as an elven word for this kind of adamant. Also, in the Marvel universe, it's made of an unobtainium called Vibronium when alloyed with steel. So, literally, while unobtainium is whatever you want it to be, adamantium has to be indestructible, or else it isn't adamantium.The Lugz said:Adamantium is a fictitious indestructible metal alloy.Nieroshai said:Adamantium. Different concept.
which is the definition of Unobtainium by default
any substance with impossible characteristics, used to fill a specific theoretical role
quite the same concept, in fact