This is about the term. Impossible materials have been used at least since H.G. Well's cavorite.

year author title term quote
2013 David Brin Startide Rising unobtainium

None of the other moons in the Kthsemenee system had the one attribute this one possessed: A core of almost one percent unobtainium.

(ch. 16, p. 93)
$Date: 2014-05-17 18:36:29 +0200 (Sat, 17 May 2014) $