Kryptonite appears as a green crystal. It is a trans-uranium element designated under the Atomic Number 137. It is not harmful to non-Kryptonians if they are only briefly exposed, but extensive exposure may cause carcinogenic harm. In its refined form, Kryptonite is considerably stronger, comparable in strength to weapons-grade uranium or plutonium. Although, in its unique property, Kryptonite is non-fissile and cannot be made into a fission weapon. However, its emission is usable to be use as a beam-weapon.
When Krypton exploded, its pieces created Kryptonite which orbited around the remains of the planet, forming a large asteroid belt. Several pieces of Kryptonite followed Kal-El's spacecraft to Earth, where they embedded in the outskirts of Smallville, Kansas.
The Kryptonite fragments were recovered by the United States government and, while having no use for the materials, had them placed into storage. Doctor Doom also took the Kryptonite fragments and would years later discover Superman's weakness when exposed to the fragments. Doom supplied Kryptonite to Magneto and, secretly, the Friends of Humanity.
Magneto would use a shard of Kryptonite in an attempt to subdue Superman to force him to accept being leader of Genosha. Superman was rescued by Avalanche and the Summers brothers, who isolated the Kryptonite, and he disposed of it by hurling it into the sun.
The Friends of Humanity used Kryptonite shards as a means to enhance soldiers to combat mutants. Of the twenty who volunteered, only four survived the enhancements, although after a single confrontation against Superman and his companions, one of them quickly died of a cerebral hemorrage, and the others would follow suit shortly after. The last sample was used to power a Sentinel which would later be used to combat Superman, although it failed thanks to his Nano-suit.
Shinobi Shaw came into possesion of a Kryptonite sample he used to craft a dagger, which he would later use to almost-fatally wound Superman in a confrontation, but Superman was saved by Power Girl, who managed to remove the blade and throw it away from him.
When General Zod attempted to take over Earth, Nick Fury decided to load a nuclear missile with the Kryptonite S.H.I.E.L.D had in storage in order to neutralize him, but sent the launching codes to General Lassider, who decided to launch the missile on his own authority even after President Obama ordered him not to, fully knowing that Superman and the Justice League had managed to lure Zod into a trap to de-power him and that such an impact would still case civilian casualties. Superman was forced to order Sentrius to open up a Phantom Zone singularity to absorb the impact's energy and prevent casualties, as well as force Zod to choose into going back to the Phantom Zone to survive, or stay outside and die with the impact.