Jewel Kryptonite is a shattered piece of the Jewel Mountains of Krypton. The Jewel Mountains were created from massive crystalline birds, who came to the mountain range to die. Their skeletons would form the Jewel Mountains over millennia.
Superman first encountered Jewel Kryptonite when he took Phantom Zone prisoner Jax-Ur into Krypton's past to find a cure for a plague sweeping Atlantis. Whilst there, they go to Jax-Ur's base hidden under the Jewel Mountains. Jax-Ur surreptitiously exposes a chiseled out piece of the Jewel Mountains to secret ray beams that change his radioactive properties. He also calculates the piece will be thrown into Earth's orbit.
Back in the present, Jax-Ur retrieves the Jewel Kryptonite. Hypnotizing Jimmy Olsen, he has Jimmy expose Superman to the strange Kryptonite. After being thrown back into the Phantom Zone, Jax-Ur uses the psychic magnifying abilites of the Jewel Kryptonite to focus his psychic energy into a beam that he detonates explosives with. He convinces Superman that detonating explosives is the side effect of Jewel Kryptonite, However, Lori Lemaris informs Superman of the truth and he throws the Jewel Kryptonite into the sun.
Post-Crisis, Jewel Kryptonite appears as a piece of a machine Agamemno needs for universal domination, alongside the Absorbascon and the Central Power Battery. An ersatz Justice League consisting of Lex Luthor in Superman's body, Black Manta in Aquaman's, Chronos in Atom's and Felix Faust in Green Arrow's travel to a distant planet to find the last surviving piece of the Jewel Mountains, in a bottled city of Daxamites Brainiac collected. Chronos-Atom shrinks down into the city to collect it. Chronos contemplates saving the shrunken city before the other villains crush it and kill the inhabitants. The piece is later substituted by Luthor with a diamond, defeating Agamemno.
In Alternate Continuities
A piece of Jewel Kryptonite, presumably artificial, appears in the super memorabilia restaurant Planet Krypton on Earth-22.