[Bobalu]

unobtanium (fuel from the movie Avatar)

In the film Avatar, unobtanium was not a fuel, it was a room-temperature super-conductor.

[lbryce]

I won't quibble with you but the definition of unobtanium allows it to be described as a fuel (any extremely rare, costly or impossible material) a substance that can be anything you want it to be, of infinite characteristics in serving whatever science fiction plot you've got going.

Wikipedia:Unobtanium

In engineering, fiction, and thought experiments, unobtainium (also spelled unobtanium) is any extremely rare, costly, or impossible material, or (less commonly) device needed to fulfill a given design for a given application. The properties of any particular unobtainium depend on the intended use. For example, a pulley made of unobtainium might be massless and frictionless; however, if used in a nuclear rocket, unobtainium would be light, strong at high temperatures, and resistant to radiation damage. The concept of unobtainium is often applied flippantly or humorously. The word unobtainium is derived from unobtainable + -ium (the suffix for a number of elements). It pre-dates the similar-sounding IUPAC systematic element names, such as Ununoctium.

*SNIP*

Science Fiction
Unobtainium can refer to any substance needed to build some device critical to the plot of a science fiction story, but which does not exist in the universe as we know it. A hull material that gets stronger with pressure in the film The Core was nicknamed unobtainium, but the concept under different names can be seen in the anti-gravity material cavorite and the super-strong material scrith from Larry Niven's novel Ringworld, which requires a tensile strength on the order of the forces binding an atomic nucleus together. Unobtainium can also refer to any rare but desirable material used to motivate a conflict over its possession, making it a MacGuffin (it appears in the story as something to obtain, not something that is significantly used). An example is unobtanium (sic - per traditional element naming not "unobtainium"[12]) in the film Avatar, a mineral valued at "20 million a kilo". Unobtainium can be used in a disparaging context (e.g., "That idea is silly; you'd need unobtainium wires to hold the planet up!") or a hypothetical one ("If one were to build an unobtainium shell around a black hole's event horizon, what would happen to the material piling up on it?").