The Infinity Stones have become the central point of the entire Marvel Cinematic Universe. These ancient items of power as old as the universe itself have been showing up for years, and will eventually be brought together, presumably by the mad Titan named Thanos. That purple-faced, crumpled-chinned monster, in the comics, gathers the Infinity Stones (or gems) together for one purpose: to end all life in the universe. Not good.
So far, we've seen four out of six of the stones, in the form of the Tesseract, the Aether, the Orb, and Loki's Staff. That means there are two remaining, and Kevin Feige, Marvel Studios President, says we'll be seeing them soon.
"You will see the other two sometime in Phase 3, for sure; there's a gauntlet that needs to be filled!" Feige said, speaking at a press conference Q&A for the release of Avengers: Age of Ultron on Blu-ray and DVD. The stones came into the equation "in the middle of phase one," he said. The use of the Tesseract, then, as the first of the Infinity Stones, was planned as early as its first appearance.
The Infinity Gauntlet, wielded by Thanos to allow him to control all six stones, was last seen in the post-credits sequence for Age of Ultron. The stones revealed so far, as confirmed by Marvel, are:
Tesseract: Space Stone, Aether: Reality Stone, Orb: Power Stone, Loki's Staff/Vision's Head: Mind Stone
That leaves Time and Soul left to reveal, and we now know those will come, for sure, ahead of Avengers: Infinity War. That means Captain America: Civil War, Doctor Strange, Guardians of the Galaxy 2, and Thor: Ragnarok are all contenders. The as-yet untitled Sony/Marvel collaboration on Spider-Man is also a long shot, now that the character will exist in the MCU.