The Infinity Stones will be hugely important in Marvel's upcoming movies. Here's a guide.

This article deals with major spoilers for Avengers: Age of Ultron.

Throughout the last few years, Marvel's movies have repeatedly teased the existence of mysterious jewels called the Infinity Stones, and Avengers: Age of Ultron contains the biggest clue yet — Thor (Chris Hemsworth) has a vision of the gems, their destruction, and the power they are capable of producing. Plus those who stayed to watch the film's mid-credits scene saw Thanos vow to procure the stones himself.

To casual fans, these shiny rocks appear to just be magic MacGuffins, side quests intended to occupy our heroes until they battle their next villain. But there's more to the Infinity Stones than meets the eye. And it's something only comic book readers will understand. Here's a brief guide:

What are the Infinity Stones?

In the mythology of the Marvel Cinematic Universe, the Infinity Stones are six "ingots" of immense power. We learn this in Guardians of the Galaxy, when the Collector explains to the Guardians (and the audience) why they're such a big deal. He says:

Oh, my new friends, before creation itself, there were six singularities, then the universe exploded into existence and the remnants of this system were forged into concentrated ingots... Infinity Stones.

The Collector goes on to say that the gems are so powerful, the people who hold them eventually blow up and die. Later we find out that you have to be special and superpowered just to handle them.

Prior to Age of Ultron, Guardians and the Collector had been our primary source of information about the Infinity Stones. But Ultron has provided us with some important new intel, by way of direct references to the stones and Thor's visit to a magical, dream-inducing hot spring. Thor's resulting vision reveals the Infinity Stones (we had never seen them together) as well as Infinity Stones in two different objects — the Tesseract (from The Avengers, Thor, and Captain America: The First Avenger) and the Aether (Thor: The Dark World).

Outside of Guardians and Ultron, however, the Infinity Stones have always lingered in secondary plots, serving more or less as an Easter egg for Marvel comic book readers. It's when you combine their role in the comic books with their depiction in Marvel's movies that things get really interesting.

Each Infinity Stone does something different, and together, they're basically unstoppable

During one scene in Age of Ultron, we're told that Vision possesses the Mind stone. It's one of the rare instances in which a character in one of Marvel's films has specifically mentioned and differentiated these coveted ingots from one another.

Usually, that kind of commentary comes from producers, directors, or Marvel Studios president Kevin Feige, who confirmed in 2013, that the Tesseract is the Space stone. The next year, Guardians of the Galaxy director James Gunn confirmed that the stone in his movie was indeed the Power stone.

All three of these references are from the comic books, where the Infinity Stones are known as Infinity Gems and have special names:

The colors of the individual Infinity Stones/Gems have changed throughout the years, including when they began to appear in the movies; what matters are their names, which spell out what each gem can do. The Reality Gem allows its possessor to alter reality, while the Time Gem affords the ability to freeze, slow, and speed up time itself. The Power Gem, as you may have guessed, yields immense power, while the Mind Gem boosts its holder's existing mental capabilities, sometimes resulting in augmented telepathy and telekinesis. The Soul Gem, according to Marvel, allows its bearer to use the gem to absorb souls. And the Space Gem enables intergalactic travel and, at its full potential, omnipresence.

In short, the Infinity Stones are awesome. If you're a superhero or supervillain, they'll make you even more powerful than you already are. And if you manage to collect and combine them all, you have what's called the Infinity Gauntlet, which will grant you unparalleled omnipotence. So if the wrong person — say, someone like Thanos — gets his or her hands on all six, you should probably just make plans to live in a different universe.

Why are the Infinity Stones important?

What's sort of strange and daring about Marvel's decision to incorporate the Infinity Stones into its movies is that neither the Infinity Stones nor Thanos are particularly integral to the story of the Avengers. Thanos is probably in the ballpark of the top seven Avengers villains, but characters like Kang, Ultron, Loki, Dr. Doom (whose movie rights belong to Fox), and even the Skrulls are all more compelling.

Part of the reason is that Thanos operates in Marvel's cosmic universe. It's a weird place with characters like Uatu the Watcher, the Acanti (a race of whale-like creatures), Galactus the Devourer, the Inhumans, Kree and Skrulls, the Silver Surfer (whose movie rights, like Doom's, belong to Fox), and a slew of other beings, races, and civilizations.

Marvel loved building out that robust world in the comics. But it raised a continual logistical question: if said world is so big and different characters all have their own adventures (characters like the Silver Surfer and Adam Warlock each had their own comic books), how do you get them to interact with Earth and the heroes who live there?

In order to set Thanos's sights on Earth, Marvel's writers would have to construct plots wrapped around space invasions or some kind of planetary threat. The Infinity Stones resolve this problem by luring Thanos from the vastness of space to experience his own earthbound adventures.

In the comics, the Infinity Stones — which, again, were known in print as Infinity Gems — played a major role in Marvel's Infinity Gauntlet series in 1991 by writer Jim Starlin and artist George Perez. Perez's art is complex, unwieldy, and psychedelic at times, but it matches up well with the overall story. Death was apparently upset that there were more living people than dead ones, so she called upon Thanos to balance the scales and start killing things:

Thanos was the perfect guy for this. He's one nihilistic bastard, and with the Infinity Gems, he was poised to wipe out vast numbers of people without even blinking:

Perez draws Thanos as a brute. His chin is square, large enough to have multiple clefts. His eyes are black orbs. He's seemingly as wide as he is tall. And his legs are thick, as if he's smuggled oil barrels under his quads. There is no elegance about him — he's a monster you send in when you don't need finesse. There is no flair with Thanos, only brutal efficiency. And in order to defeat him, Marvel's greatest superheroes must come together knowing full well that they probably don't stand a chance of winning:

This army of heroes and how easily they're picked apart underscores Thanos's true power. If it were just the Avengers or just Thor going out into space and fighting him, we wouldn't experience the same kind of gravitas. Beating Thanos will require assistance from a being who's stronger than him (which is basically nonexistent); truthfully, the only thing powerful enough to beat Thanos is Thanos.

The latter matchup comes to fruition as the Infinity Gauntlet is assembled. Something strange happens to this undulating hunk of evil, and Thanos gets thirsty. The Mad Titan™ isn't just content to do what Death asks him to do. He wants to impress her and earn her love. Nothing in this moment is more important to Thanos than Death's affection.

But Death is stone cold, and puts Thanos in the friend zone, a friend zone from which he never escapes:

Starlin writes this sense of inadequacy as Thanos's fatal flaw. He becomes so laser-focused on making Death his girlfriend that he starts doing dumb things like acting weak in hopes of getting Death's attention. Terminal thirstiness is Thanos's only character trait, other than his bleak nihilism:

Starlin and Perez's comic eventually finds order restored and Thanos becoming a farmer, but not before floating the idea that the Infinity Gems/Stones are limited by whoever holds them. Despite his bloodlust, Thanos was defeated because he was careless. We see his granddaughter Nebula (in the movies, she's his adopted daughter) try — and fail — to control the gauntlet. And in the end, we see a character named Adam Warlock promise to distribute the Infinity Gems among people he can trust.

What happens to people who possess an Infinity Stone/Gem?

There's something Tolkien-esque about the dynamic between the people who possess the Infinity Gems. Like the Ring, nefarious people want to control them. And there's always the lingering suspicion that someone may want to orchestrate a power play to try to obtain the other gems.

Moondragon (the salty bald woman pictured above) has the Mind gem, and it enhances her psionic powers greatly. She's a telepath, but when Warlock gives her the Mind gem, she gains telekinetic powers. That's a major power boost. But in order to keep her in check, Warlock installs safeguards between Moondragon and the rest of gem holders, a group called the Infinity Watch:

Throughout comic book history, the Infinity Gems frequently change hands. They eventually land in the possession of a group called the Illuminati, a group of "good" superheroes — Dr. Strange (Soul), Reed Richards (Reality), Tony Stark (Space), Professor Charles Xavier (Mind), Captain America (Time), Namor (Power), Black Bolt, and Black Panther — that you would theoretically trust with them (assuming someone has to possess them). The Illuminati keep each other in check, and they keep the Infinity Gems hidden from one another.

The Illuminati's Infinity Gems are the main focus of the Infinity arc in Jonathan Hickman's New Avengers. Hickman's story is punctuated by the clashing members of the Illuminati, but also the hesitation with which they regard the gems. Unlike Thanos, they're reluctant to use these powerful objects, and the gems represent a last resort rather than a desired MacGuffin:

What is currently happening with the Infinity Gems in the comic books?

In the aforementioned Hickman arc, Captain America assembles the gauntlet to ostensibly save the world. And in the process, five of the six gems are destroyed. Meanwhile, the Time Gem straight up disappears:

According to Marvel, the Infinity Gems/Stones will appear in the upcoming Marvel comic book event Secret Wars.

Will the Infinity Stones meet the same fate in the movies?

Maybe. But if the Infinity Stones are going to be destroyed, their obliteration might not occur in the same fashion. Hickman's Avengers run is grand, expansive, and interconnected. It's a complicated, rewarding beast that involves different universes, journeys into the cosmos, and dozens of new characters. Though not impossible, it would be a massive undertaking to bring all of the complexities of Hickman's run to the big screen by the time Avengers: Infinity War Part I is released in 2018.

What the movies will possibly try to do is combine elements from different comic arcs throughout history. I suspect Infinity War will draw from the comic book The Thanos Quest, a two-issue comic book that sees Thanos hunting down powerful, immortal beings in order to assemble the Infinity Gauntlet:

That falls in line with the movies, and especially the mid-credits scene from Avengers: Age of Ultron, in which Thanos says he's going to assemble the gauntlet himself. In the Marvel Cinematic Universe, Nova Prime, the Collector, the Asgardians, and Vision currently hold four of the six Infinity Stones (remember, two of them have yet to be revealed) — and Thanos will probably hunt down these people and organizations to assemble his gauntlet.

The other comic book arc Marvel Studios might borrow from is the company's recent Infinity arc. That arc featured, among other things, Thanos trying to kill his long-lost son, some of the most powerful Avengers traveling to space to deal with aliens, Thanos attacking Earth while those Avengers were away, and the detonation of a Terrigen bomb that evolved some of Earth's population into Inhumans. It may have been streamlined, but it hits all the marks of what Marvel wants to do with its characters and storylines in Avengers: Infinity War, namely:

• Going to space will allow for a Guardians of the Galaxy crossover and, if need be, a Captain Marvel tie-in
• Letting the Guardians of the Galaxy play a role in helping save the universe
• Using the Terrigen bomb to set up the Inhumans movie that's slated for 2019
• Bringing Thanos to Earth
• One of the battlefields in Earth's invasion is Wakanda, where Black Panther, a new character with his own movie coming out in 2018, rules

So what happens next?

We haven't yet seen all the Infinity Stones. We're still missing Time and Soul, which should make Marvel's next few movies interesting in terms of where and when they might show up.

The next movie on the company's docket, however, is Ant-Man (July 17), which will be followed by Captain America: Civil War (May 2016). Both of those movies seem more concerned with character development and team dynamics than they do with introducing new Infinity Stones, though I could be very wrong.

If I had to guess, I'd wager that the Soul gem will probably show up in Dr. Strange (November 2016). Dr. Strange was in possession of the Soul gem in the comic books, and it fits into his mysterious and magical world. Spells, sorcerers, magic, and all-around weirdness make the idea of a soul-taking gem seem a little unless unbelievable.

The Time gem will probably find a place in either Guardians of the Galaxy 2 (May 2017) or Thor: Ragnarok (July 2017). Both movies deal with science, and the aspect of time travel has always been a fascination for sci-fi movies. The idea of time travel could also make a neat segue into spelling out Star Lord's (Chris Pratt) mysterious history.

But no matter when the final two appear, all six Infinity Stones, as well as Thanos and Thanos's Infinity Gauntlet, will undoubtedly make an appearance in the two-part Avengers: Infinity War in 2018 and 2019. Worlds will be broken, heroes will be defeated, and this epic Marvel mystery will finally be solved. And we'll all be watching.

WATCH: Welcome to the digital age of comics

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