“Fine, I’ll do it myself,” growled Thanos in a mid-credits scene after Avengers: Age of Ultron. All around the world, fans heard a collective "SPLOOSH!" as their Captain America briefs were simultaneously ruined. In that scene, the MCU’s Big Bad donned the Infinity Gauntlet, the golden glove which becomes the most powerful weapon in the universe when all six Soul Gems are collected — wait. What’s a Soul Gem?
The objects the Marvel Cinematic Universe refers to as the "Infinity Stones" were originally called "Soul Gems" in the comics, though they are now known as Infinity Gems on the printed page. According to The Collector (Benicio Del Toro) in the MCU, the Infinity Stones were created during the Big Bang, remnants of six original singularities. In the comic books, the Soul Gems were once a primordial being known as Nemesis, who ended its existence because it was lonely being the only sentient being in the universe.
So what are Infinity Stones? Regardless of which cosmic canonical origin you prescribe to, the Stones are the Stones, and the MCU has introduced five of the six. The six stones each represent one aspect of the universe: Mind, Power, Time, Space, Reality, and Soul. Whoever holds a stone has total and universal control over its corresponding aspect. Long story short, it’s probably not ideal that a guy nicknamed the “Mad Titan” is trying to collect them.
While the Infinity Stones of the MCU are essentially the same as the Infinity Gems of the source material, there are notable differences beyond the flip-flopping and swip-swapping of colors. So, let’s untangle this knot and figure out the differences between the Soul Gems and the Infinity Stones.
The Space Stone was the first Infinty Stone introduced in the MCU and has appeared in multiple films so far. Technically, it made its debut in Thor, specifically in the post-credits scene in which Loki gets his hands on it through Dr. Selvig. The Stone was referred to as the Tesseract. Whether that's the name of the stone itself or the blue cube in which it's contained is unclear, but it allows the wielder to cross any space instantaneously. It was the subject of Captain America: The First Avenger, and was last seen in The Avengers when Thor used it to take Loki back to Asgard in his custody, where it presumably remains.
In the comics the Space Gem is purple. Like in the movies, it allows the user to travel any distance or move objects across space and time instantaneously. At the height of its powers, it grants literal omnipresence: the state of being everywhere at once. More mundanely, it grants the wielder extra speed, which we'll likely see on display in Avengers: Infinity War. The Space Gem first appeared in Avengers Annual #7 (1977).
Ironically, since it is the only Stone yet to make an appearance in the MCU, the Soul Gem was the first Stone introduced in the comics in Marvel Premiere #1 (1972). The most interesting thing about the Soul Gem is that it is sentient. It actively hungers for souls and can steal and control them (living or dead). It also allows the wielder to steal the powers or abilities of their victims.
In the comics, the Soul Stone has a pocket universe within it known as Soul World, an idyllic and peaceful paradise in which the original keeper of the Stone, Adam Warlock, would trap rowdy folks, and occasionally enter himself. Well, technically the High Evolutionary was the first keeper of the Stone, but he gave it to Adam Warlock for protection. We can’t yet say what the powers of the Soul Stone will be in the MCU, but we at least know that it will differ in color. In the books the Soul Stone is green, which obviously it can't be in the movies as the Time Stone already claimed that color. If the MCU follows the color conventions of the original Gems, the Soul Stone will be orange.
The Mind Stone was rather confusingly introduced in The Avengers, stored in a blue shell on the end of Loki's staff. It turned out the Stone itself was actually yellow, which we didn't discover until Avengers: Age of Ultron, finally putting to bed some really confusing questions about the Space Stone and the Mind Stone being fused in the MCU. The Mind Stone brought the Vision to life and now rests securely in his forehead, which hopefully remains the case for his sake. Thanos, however, is probably going to be the cause of a really bad day for our friend the Vision.
The Mind Gem actually was blue in the comics (compounding the aforementioned confusion) and first appeared in Avengers Annual #7 (1977). It was the manifestation of the universe's collective subconscious, granting the user enhanced mental and psionic abilities, as well as granting them access to the thoughts and dreams of others. When used in tandem with the Power Gem it allowed the wielder access to "all minds in existence simultaneously."
With the Reality Stone, the MCU took perhaps the most liberty from the source material, introducing the object in Thor: The Dark World as the Aether, a mercurial, red plasma-like substance constantly changing shape. In the movie, it was a little unclear what it did: change matter into dark matter? Make the wielder generally stronger? Make Kat Dennings's once loveable character super annoying? Regardless, it was given to the Collector in the post-credits scene, but his museum has since been destroyed, so its current whereabouts are unknown.
The Reality Gem is yellow in the comics, first appearing in Avengers Annual #7 (1977). The Reality Gem grants wishes, in a way: it can provide literally anything to the wielder, regardless of physical laws. It can create an alternate reality for the user and, in concert with the other Stones, at its full power can change the reality of the entire universe.