Gems of Cronus

The Gems of Cronus were three powerful jewels that were created by the ruler of the Titans, Cronus. 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 those gems, Cronus could control all of reality and bend it to his will. Due to that great power, Cronus went mad with power and paranoia, and nearly destroyed everything, had it not been for his son, Zeus, who then defeated him. Cronus was then stripped of the Gems and they were cast down to Earth in order to keep them out of his deranged hands.

Jack tried to obtain the gems when he heard that they could help him go back in time, due to the fact that Cronus had control over human time, by a mysterious Hermit. After Jack obtained the three gems, the Hermit revealed himself as Aku, used the gems, and revived Cronus. However, Jack knew that it was Aku the whole time, and kept one of the jewels, which weakened the pseudo-Titan and destroyed him. 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 destruction in the past, the Gems of Cronus possibly remained in their hiding place in the future.

Gems

 * The Eye of Cronus: The gem that was placed in Cronus' Helmet. That gem gave Cronus the ability to focus his vision of universal time.
 * The Fists of Might and Ability: The two gems that were placed in Cronus' Gauntlets. Those gems gave Cronus the power to grasp the cosmic essence and mold it to his liking.

Trivia

 * After their introduction, Jack never used the Gems of Cronus after he obtained them or even mentioned them. It could be assumed that Jack lacked the magical knowledge and capability to use the Gems of Cronus or they were useless without the Helmet and Gauntlets, which meant that the Gems were useless to Jack. It was possible that Jack could have sold the Gems of Cronus in order to get some money for supplies.
 * It was possible that the Gems of Cronus were an homage to the lnfinity Gems from the Marvel Universe: those Gems gave the user unfathomable power and were sought out by many villains, most notably, Thanos.

Appearances

 * Episode XXXIV: Jack and the Swamp Monster