The Infinity Stones have not only become a central plot point of the Marvel Cinematic Universe, they have repeatedly made their presence known in the comic book universe as well, and they have been the obsession of Thanos, the mad Titan, all because he is basically in love with “Death,” and we’re not talking about death as in dying, but the actual personification of Death, and that is why he has been on this mad quest to collect all of the Infinity Stones (or gems).
So far we have had the Tesseract/Space Stone (Captain America, The Avengers), Aether/Reality Stone (Thor: The Dark World), the Orb/Power Stone (Guardians Of The Galaxy), and Loki’s Staff/Vision’s Head Gem/Mind Stone (The Avengers, Avengers: Age Of Ultron). What this means, of course, is that there are still two remaining, and only Kevin Feige is the one with the true vision to know when that will be.
“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, which is worn by Thanos, allows him to control all six stones, and based on what was seen in the post-credit sequence for Age Of Ultron, it’s clear that the remaining two stones will show up.
The two remaining stones to appear are Time and Soul, but given some of the Marvel titles that are to slated to show up soon, most notably Doctor Strange, Guardians Of The Galaxy 2, and Thor: Ragnarok, it’s a pretty safe bet that given the nature of those movies that the remaining gems will appear in at least one of them, which will eventually lead into Avengers: Infinity War.