Kryptonite is a fictional element found in DC comics, originally it came in many varieties but by far the most well-known today is Green Kryptonite - which robs Superman of his superhuman abilities and can even kill the legendary "Man of Steel" if he is exposed to the unusual radiation it emits for too long.

Kryptonite is also one of the few things known that can cut Superman's skin, as was seen in the movie Superman Returns - in which Lex Luthor created Kryptonite daggers for use against Superman.

Kryptonite can also be used as an energy-source, such as the artificial heart of Metallo, however it is shown to be radioactive and can cause poisoning in humans (as happened to Lex Luthor in Justice League Unlimited).