The Gems of Cronus were three powerful jewels created by the ruler of the Titans, Cronus and they appeared in episode XXXIV: Jack and the Swamp Monster. Each Gem gave Cronus a power and he placed each into a piece of armor; one in his helmet and two in his gauntlets. With these gems, Cronus could control all of reality and bend it to his will. Due to this great power, Cronus went mad with paranoia and power, nearly destroying everything had it not been for his son Zeus defeating him. He was then stripped of the Gems and they were cast down to Earth to keep them out of the hands of his deranged father.
Jack tried to obtain the gems when he heard that they could help him go back in time, due to the fact Cronus had control over human time, by a mysterious hermit. After obtaining the three gems, the hermit revealed himself as Aku and used the gems to revive Cronus. However, Jack knew it was Aku the whole time, and kept one of the jewels, weakening the pseudo-Titan and destroying it. Aku then tried to use the power himself, but was defeated and ran away. Jack then gathered the three gems and went on his way.
After Aku's demise, the Gems of Cronus are still being sealed in their prison since Aku was not there to trick Jack to help him retrieve them.
- The Eye of Cronus: The gem placed in Cronus' helmet. This gem gave Cronus the ability to focus his vision of universal time.
- The Fists of Might and Ability: The two gems placed in Cronus' gauntlets. These gave him the power to grasp the cosmic essence and mold it to his liking.
- After their introduction, Jack never used the Gems after obtaining them or even mentioned them. It can be assumed that Jack lacks the magical knowledge and capability to use the Gems, so the Gems are useless to him now.
- It is possible that the Gems of Cronus are an homage to the lnfinity Gems from the Marvel Universe. Gems that gave the user unfathomable power and are sought out by many villains. Most notably Thanos.