Since Avengers: Infinity War is poised be the biggest movie in the history of everything, and the entire plot hinges on the immense importance of the so-called "infinity stones," a neglected question arises.
What are they?
Well, to the dramaturge, they are a transparently arbitrary plot device used to set the plot in motion, to explain why the protagonists are battling the antagonists. The fate of the universe rests entirely on control of (fill in the blank). Could be anything: infinity mood rings, infinity cabbage patch dolls, infinity marbles, infinity fuzzy dice.
Marvel picked stones.
And the Marvel Cinematic Universe is not the only franchise guilty of using them. Remember the Lost Seeing Stones of Lord of the Rings? Gandalf gets hysterical when he sees one rolling across the floor, throwing a cloak over it and saying, portentously, as if it's some major plot point, "They are not all accounted for, the Lost Seeing Stones. We don't know who else may be watching!"
Sure we do. Saruman and Sauron, who already know what's going on anyway. We wait for the important "accounting" of the lost stones, but it never happens, and at any rate has no real bearing on the ultimate outcome of the story. When you have interesting characters, nobody really cares.
Marvel isn't even the only superhero franchise that relies on these MacGuffins. The Mother Boxes of the unlamented DC Universe movie Justice League. An evildoer wants to take control of mystically powerful Mother Boxes, and the Justice League superheroes must stop him. Side note: Viewers who endured the awfulness of Justice League sometimes modified the name of the Mother Boxes by inserting a two-syllable gerund between "Mother" and "Boxes."
The Mother Boxes serve the same basic narrative purpose as the infinity stones. Someone bad — in Infinity War's case, it's Thanos — wants them, and all the good guys — The Avengers — must coalesce to prevent this from happening.
The infinity stones, though, are painted with a much thicker coat of mythology, with lots more narrative sparkles and glitter and pony stickers. There are twice as many stones as boxes (six to three), and the MCU has different names for them. Sometimes they are called the soul gems. In Infinity War, they are referred to as elemental crystals. And some of the individual stones have two names, for maximum confusion.
Over the course of the 18 Avengers movies, fans have been given clues as to the nature, location, and importance of the infinity stones. Their origin was explained by The Collector (Benicio Del Toro) in Guardians of the Galaxy Vo1. 2:
"Before creation itself, there were six singularities, then the universe exploded into existence and the remnants of this system were forged into ingots."
Singularity also refers to the relationship status of any MCU fan who already knew that.
Anyway, the ingots became the stones, and here's what we know about what they are, what they're called, and how they've been passed around and acquired by Marvel characters:
The Space Stone (also known as Tesseract): A blue stone that can transport you around the universe or create super-weapons. Captain America had it, then S.H.I.E.L.D., then Loki, then Thor, then Loki again at the end of Ragnarok.
The Reality Stone (also known as Aether): Can exist in the form of a dangerous red liquid (like Jägermeister). Natalie Portman was infused with it for a while in Thor: The Dark World, but was last in the hands of The Collector.
The Power Stone (also known as Orb): Purple stone that gives the user super strength. Chris Pratt had it in Guardians of the Galaxy, but gave it to the Nova Corps (outer space law enforcement).
The Mind Stone: Once owned by Thanos, who lent it to Loki for his magic scepter. It's a bauble that confers the power of illusion and mind control. It went from him to S.H.I.E.L.D. to HYDRA, and in Age of Ultron passed to Vision (the Paul Bettany character), and now it's part of his head.
The Soul Stone: Mentioned in Doctor Strange as perhaps the most powerful of the stones, but yet to make an appearance in the movies. Expect big things.
The Time Stone: A green rock, it gives Doctor Strange the power to manipulate time. He keeps it in the Eye of Agamotto, which in turn is kept in his New York City pad.