Kryptonite is a meteorite rock commonly used in Lois and Clark: The New Adventures of Superman.

It is mainly used as a weapon against Superman either to kill him or alter his personality and powers.

When Superman's home planet Krypton exploded, fragments of the radioactive world came to Earth and scattered around the planet. After finding out that these meteorites came from Krypton, they named it "Kryptonite".

Two varieties of the substance were found, both had different side effects on the Man of Steel, but were still deadly:

Green Kryptonite

Clark exposed to Green Kryptonite

Clark exposed to the Green Kryptonite

The most common variety that can painfully weaken Superman or even kill him when he is exposed to it.

It was first discovered by Wayne Irig, a friend of the Kents who gave it to them to keep safe, but when they showed it to Clark, it had painful effects on him, making them discover how deadly it was and vowed never to expose their son to it again.

The first villain to know about it was Jason Trask who stole the substance and used it to kill Superman, but failed. (The Green, Green Glow of Home)

Many other villains have tried, some using the traditional meteorite method, while others had used a far more creative way. Some very well nearly succeeded, but Superman always found a way to save himself:

However it once even helped him be cured of a Kryptonian virus (Home Is Where the Hurt Is) and can possibly counteract an exposure to another substance of the meteorite.

Red Kryptonite

Red Kryptonite

Gene Newtrich finding Red Kryptonite

This variety was different in colour but still deadly, it made Superman apathetic and also made him unable to control his powers.

It was first discovered by scientist Gene Newtrich who told the new head of Intergang, Bill Church, Jr. about it and tried to use it against the Man of Steel but failed. (Individual Responsibility)

Later, Gene's two sisters Lucille and Nell built a laser to make Superman permanently apathetic, but only succeeded in transferring his powers to Lois Lane, however he got his powers back and disposed of the device. (Ultra Woman)

It was used again by electronics madman Mr. Gadget who used Perry White's own son Jerry to expose Superman to it and make him unable to control his powers. But eventually Superman got control of his powers again especially when a green Kryptonite bullet grazed him counteracting his exposure to the red variety. (Lethal Weapon)