Jewel Kryptonite was a variant of Kryptonite.
Jewel Kryptonite was created from the remnants of Krypton's Jewel Mountains. It allowed the Phantom Zoners to heighten and focus their low-level mental abilities in order to affect the universe outside of the Phantom Zone.
Jewel Kryptonite was the name given to the remnants of Krypton's Jewel Mountains. In their search for the three components to the ultimate weapon, Agamemno and the Injustice League (who swapped bodies with the members of the Happy Harbor-era JLA) seized a fragment of Jewel Kryptonite from one of Brainiac's marooned ships on an aquatic alien world. The purpose of the Jewel Kryptonite was to combine with the Green Lantern Central Power Battery, another stolen component of the ultimate weapon, in order to neutralize its weakness against the color yellow. There is no known connection to the Phantom Zone.