Not of This Earth

"I don't know; Therefore Aliens"

- Ancient Aliens Meme

Physical proof of alien activity.

An object of indeterminate origin has been found. It is sent to a lab for analysis, but the lab cannot identify the material it's made of. Similarly, when the heroes ask the lab to identify the mysterious glowing substance found at the crime scene, the reply may come back: organic-based, but unknown. Sometimes the scientists go so far as to say conclusively that whatever it is, it cannot be found on Earth.

Such substances are something of a MacGuffin. Often they do not possess any special properties or behaviors, although at times they are found to be Unobtainium. The purpose of declaring a substance to be Not of This Earth is to "prove" extraterrestrial origin or activity.

To handle this trope plausibly, the substance should be made of known elements but perhaps in isotope ratios not typical of Earth or the Solar System, or else it should be a manufactured substance using techniques beyond Earthly science.

Anime and Manga

 * Not quite aliens, but the trope comes up in Death Note. The police get their hands on a Death Note after the Yotsuba arc, and their lab reports state that the paper matches no known earthly substance.
 * Darker Than Black has a lot of this. Besides the Alien Sky and Differently Powered Individuals that suddenly appeared with the Gates, some really weird crap comes out of said Gates. Like stone flowers that grow and can be used to make a drug, a crystal that makes Contractors more powerful, or technology that lets the user alter memories. Even the scientists studying them occasionally have to just give up and say that Reality Is Out to Lunch.
 * Witchblade is a strange case. In the original, Witchblade was presented as a reproducing creature (it has parents). Shapeshifting, tank-ripping symbionts have few in common with Terran life. So the creators of the adaptation surrounded it with some alien flora in Dream Within a Dream episode endings -- and it doesn't look out of place there.

Film
"What is this strange metal? It is neither brass nor iron. It is like no metal that I have ever seen... By the gods!... I was right to say "by the gods". Who else could make a sword that slices through solid marble, without leaving the slightest blemish on the blade?"
 * In Superman III, Richard Pryor's character does an analysis of kryptonite, and the results indicate that a certain percentage of it is simply "unknown".
 * In Predator 2 the creature's weapons are made of metal that doesn't fit in the description of any known element.
 * In Clash of the Titans, Ammon says of Perseus's sword:

Literature

 * One of the earliest examples is The Colour Out of Space by H.P Lovecraft -- where the mysterious Green Rocks are not only an alloy unknown on earth, but of a color unknown on earth.
 * An idol depicting the eponymous Eldritch Abomination appears in Call of Cthulhu; it's made out of stone that scientists are unable to identify. The matter from which it was made was brought to Earth by the Great Old Ones.
 * in Sphere, a sample of the hull of a supposed alien ship is analyzed and found to be made of common elements, but they've been worked into a composite form that nobody yet knows how to duplicate.
 * The metal used to make the special swords in Highlander had a similar moment when a scientist studied them and determined there was "no metal that behaved like this" (paraphrased).
 * Plutonium-186 in the Isaac Asimov novel The Gods Themselves. Chemists and physicists are befuddled by an isotope of plutonium whose nuclei shouldn't hold together for more than a trillionth of a trillionth of a second, according to the natural laws of our universe. Then, one scientist realizes that the reason plutonium-186 can be stable is because it brought a bit of the physics of its home universe with it.
 * Roadside Picnic has Alienated Zones peppered with really strange... things. And effects. From common "empties" (a pair of parallel copper disks with wide gap between, acting as one solid body) and "black spray" (beads that noticeably delay and red-shift light) to fog that turns your bones into jelly, and weirder. Some of them were even removed and used outside... and remain Black Box stuff that even after being intensely researched affects science mostly by demonstrating that objects with such outrageous properties can exist. One of theories is that it's a load of alien trash - random, mostly broken, deteriorated or used up stuff tossed away - so of course for humans it makes as much sense together or separately as junk left by a bunch of tourists for ants (hence the title).

Live Action TV
"House: The results came back. The lab cannot identify the metal. Said it might not even be terrestrial.
 * Subversion: In the House episode "Cane and Able," Chase removes a small piece of metal from the shoulder of a boy who claims he has been abducted by aliens.

Dr. Chase: Really?

House: No, you idiot. It's titanium."

"Doctor: (Removing a larval Goa'uld) It's not human!
 * Cleverly averted in an episode of The Equalizer. The title character helps a street crazy who claims to be an alien; it turns out the man is actually a brilliant research scientist who presumably snapped when corrupt execs forced his invention away from him... however, at the end, he's disappeared, and it's discovered that the metal medallion he left behind "was manufactured in a vacuum." "You mean," the Equalizer demands, "this thing was made in outer space?" His assistant shrugs and grins. "They just don't know."
 * Though humans make this in vacuum without going into space with some regularity.
 * The odd watches carried by angels on Port Charles. Victor's lab cannot identify the metal.
 * Whenever silicon-based biotic material is mentioned, or DNA containing more than four bases (e.g. The X-Files).
 * Done not with a metal, but with a Jaffa in the pilot episode of Stargate SG-1.

Jack O'Neill: Ya think?"


 * Though the movie, and very early episodes, did treat Naquada like this; before they learned the name, they referred to it simply as "the Stargate element," because it was unknown on Earth.
 * There is also the super-strong/light metal trinium.
 * Occurs in Doctor Who in several episodes over the series run. At one point the Third Doctor specifically was testing for something that he knew couldn't exist on earth!
 * Occurs in Smallville with kryptonite, and various Kryptonian devices and other alien materials.

Video Games

 * Subverted in Condemned 2: Criminal Origins. One of the TVs displays an interview with a scientist who has been analyzing samples of the metal that the Oro Invictus use to enhance their powers. When he explains that he cannot place the origin of the metal fragments, the interviewer immediately suggests that the metal pieces fall under this trope. However the scientist shoots this theory down, stating that the pieces are of Earthly origin; they're just made using a previously-unknown manufacturing technique.

Other

 * A standard element in alien crash cover-up conspiracy theory. Some people claim to have found some metal debris near the crash site, allegedly part of the alien ship. Said metal usually is strangely light, yet highly resistant to impact (often said to be bulletproof). To this date, no such metal has been subjected to public scrutiny.