Dr. Helen Cho: The gem: it's power is uncontainable.
That's for the tip, Doc. This sure sounds like something that deserves some attention, wouldn't you stay? She's talking about the Mind Gem—or, as it's also called in the movie, the Mind Stone (not to be confused with a rhinestone—so 1980s). It's that pretty, amber, glowing sliver of awesomeness that sits smack dab in the middle of Vision's forehead. It's a fashion-forward look for him, but it's also a potent symbol.
In terms of the broader Marvel Universe storyline, the Mind Stone is one of the six Infinity Stones that uber-baddie Thanos is trying to collect in order to wield ultimate power. Different stones feature in different storylines, but in our case we have the Mind Stone, which is hanging out inside Loki's scepter like the delicious center of a Tootsie pop. Ultron removes it from its casing in the scepter and installs it in his android prototype, which then seems to function like the ultimate Duracell battery.
It's more than just a power source, though. Age of Ultron is a movie that's really interested in notions of intelligence—particularly artificial intelligence. The Mind Stone is where the "brain" of Ultron is first hanging out. Once he flies that jeweled nest, the Stone gets transplanted into Vision and signifies his artificial origins with a big, shining beacon in the middle of his head. In this way, the Mind Stone is a pretty obvious symbol of extra-human intelligence.
Is AI a good thing or a bad thing? What are its limits? What are its possibilities? What does it mean for us squishy-brained mortals? These are all questions that Age of Ultron is asking, and the Mind Stone is the visual center of that curiosity. Like any good movie, it leaves the answers largely up to us, the viewer. On the one hand, AI spins totally out of control in the character of Ultron (see "Ultron" for the deets on him), but then again the Avengers wouldn't have defeated him without Vision, or Tony's Stark other AI helper, Friday.
So, AI: yay or nay? The question's complicated enough to give you a big, glowing headache, right in between your eyes.