[url]https://en.wikipedia.org/wiki/Unobtainium
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Unobtainium?
'In fiction, engineering, and thought experiments, unobtainium is any hypothetical, fictional, or impossible material, but it can also mean a tangible but extremely rare, costly, or reasonably unobtainable material. Less commonly, it can refer to a device with desirable engineering properties for an application, but which are exceedingly difficult or impossible to achieve.'
from link above.
nonextant
inaccessible