If you haven't seen Guardians of the Galaxy you don't have to worry about spoilers in this post because which stone the Orb is was never revealed in the film. Until now...
There are six Infinity gems in the Marvel comic book universe which make up the Infinity Gauntlet. The gems (and their comics corresponding color) are Power (red), Soul (green), Time (orange), Space (purple), Mind (blue) and the Reality (yellow) gem. In the Marvel Cinematic Universe, the gems have been adapted into being stones.
Marvel fans have been trying to figure out what stone represents which comic book counterpart gem and while some have tried to line up the colors of the gems to the movie stones, this is not an accurate assumption.
The Tesseract in Captain America: The First Avenger and in The Avengers has been confirmed to be the Space stone by studio president Kevin Feige and is blue in color. This is definitely not a direct representation of the gem and its color from the comics.
The Aether in Thor: The Dark World is claimed to be the Reality stone and is red in color. Some assumed this was the Power gem, due to its color, but not according to Wiki. (Yeah, I know that I'm using Wiki as a source but it's the only source claiming this outright.)
In Guardians of the Galaxy, fans have had no idea what comic gem the Orb was related to. But now, writer/director James Gunn has confirmed the Orb is indeed the Power stone.
“@sensmile11: @JamesGunn Are you allowed revealing which infinity stone was used? Loved the movie, you nailed it” Power stone.
— James Gunn (@JamesGunn) August 3, 2014
Quick breakdown of the Infinity Stones are: Tesseract is the Space stone, Aether is allegedly the Reality stone and the Orb is confirmed to be the Power stone. Meanwhile, the gem in Loki's scepter from The Avengers—and last seen in the possession of Baron Wolfgang von Strucker in the post-credits scene of Captain America: The Winter Soldier—is still an unknown factor right now. The easiest assumption to make is that it's the Mind stone because it took control of minds in The Avengers. This would leave two stones left to be uncovered—if Loki's scepter is indeed the Mind stone—and those are the Soul and Time stones.
In next summer's Avengers: Age of Ultron, there's a chance of a reveal of what the power of Loki's scepter really is and it may also be the way Quicksilver and Scarlet Witch gain their respective powers too. Or Loki's scepter is not an Infinity Stone at all. Fans can only wait and be patient until the remaining Soul, Time and Mind stones are officially revealed.
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