This is the picture of the gems that power The Infinity Gauntlet:
The gem in the scepter is considered as The Mind Gem. It does make sense that they use it on the head area instead of chest area.
In Avengers: Age of Ultron, the Korean scientest follows Ultron's orders even without the visual confirmation of Ultron's existense beside her.
Because The Mind Gem is the source of Ultron's sentience.
In the movie, Tony Stark uses Loki's Scepter to create artificial intelligence peacekeeping program (ULTRON), it backfires pretty bad but that's irrelevant. This might be the proof why the scientist follows Ultron's orders.
Mind Gem allows the user to greatly strengthen and enhance mental and psionic power and access the thoughts and dreams of other beings. Backed by the Power Gem, the Mind Gem can access all minds in existence simultaneously. When searching for it, the Grandmaster apparently discovered that the Mind Gem was the manifestation of the universal subconscious.
In the series Warlock and The Infinity Watch, the Gems are distributed by Adam Warlock to a group he dubs "the Infinity Watch", with himself as the leader and once again guardian of his old companion, the Soul Gem. He names the astral samurai Gamora guardian of the Time Gem, the hedonistic rogue Pip the Troll guardian of the Space Gem, the impervious powerhouse Drax the Destroyer guardian of the Power Gem, the Eternal-trained human mystic telepath Moondragon guardian of the Mind Gem, and an unknown member (later revealed to be Thanos himself) guardian of the Reality Gem.
Bonus Information: Some die-hard Marvel fans does not accept the stone in Loki's Scepter as The Mind Gem because they think Thanos would never give up such strong weapon (for more info please check The Infinity Gauntlet). They think the stone as something that is powered by Tesseract. But in Guardians of the Galaxy, Ronan the Accuser has The Power Gem.