Unobtainium is the name for any magic material that has precisely the properties needed to do the job; that you can't get hold of.
It has historically been used to describe titanium in the US, this is because the Russians are the major suppliers of the material and were not terribly keen to sell it to the Americans for use on military projects such as the SR-71 blackbird.
Roninspoon writes "interestingly enough, I have heard that during the SR-71 construction program, the CIA set up a fictious company overseas to buy titanium for the project from the Soviets"