The Time Gem is one of six Gems that came to be known as the Infinity Gems (previously known as "Soul Gems"). They are some of the most powerful artifacts in the Universe. Each one allows its owner complete omnipotence over one aspect of the universe; time, space, reality, mind, power, and soul. When all six are directly wielded together, nothing is impossible for their bearer.
It is said that the Gems were actually once part of the primordial entity, Nemesis, who, eons ago, ended its existence rather than live as the only sentience in the universe. However, the gems were not known to be a collective power for ages. Until Thanos, of Titan, assembled the Gems into the Infinity Gauntlet, which exalted him to the position of a Supreme Being.
When used alone, the Time Gem enabled its host the ability to travel through time, and manipulate time through acceleration or slowing it down. The Gem could be used to age another or revert them back to their infant state. While in possession of the Gem, Gamora was able to see the coming fate of Adam Warlock at the hands of the time traveling Badoon agent, Maxam. After the event known as the Illuminati, the Atlantean ruler, Namor has taken possession of the Time Gem.