Kryptonite was the name coined by Emil Hamilton for radioactive fragments of the destroyed planet Krypton. This substance had immediate, detrimental effects on Kryptonians and Argoans, as well as long-term effects on humans.
The existence of Kryptonite was first discovered shortly after Superman's first public appearance in Metropolis. Early tests were conducted by LexCorp scientists, and further tests by Professor Hamilton, who theorized that the low-level radiation generated by the element was a result of the explosion that destroyed Krypton.
The fragments found on Earth varied in size, ranging from small slivers to pieces several feet across.
Kryptonite's most noteworthy effect was sapping the strength of Kryptonians powered by a yellow sun. Prolonged exposure to its radiation could quickly lead to death. Kryopyonite also left a burning sensation, and it was the only known non-magical substance that could pierce Superman's normally invulnerable skin.
Kryptonite exerted the same malevolent effect on Argosians, such as Supergirl. However, it was harmless to Bizarro, presumably because his genetic makeup was too flawed, despite having Kryptonian DNA. He even called it "pretty" when he held it.
The radiation could also affect humans, although much longer exposure was required for them to suffer ill effects. In comparison to Superman who felt the effects almost instantly, the side effects experienced by humans could apparently vary for any number of reasons. For example, previous owners of "The Laughing Dragon" statue, carved from Kryptonite but erroneously thought to be made of jade, all died prematurely; Lex Luthor developed Kryptonite poisoning of the blood; essentially a form of cancer as a consequence of carrying a large chunk of Kryptonite around with him for years. The resulting cancer was apparently so severe that no form of treatment could be successfully provided.
Like all Earth-bound radioactive materials, Kryptonite's radiation could be blocked by lead.
Kryptonite's most frequent use was as a weapon against Superman and sometimes against other Kryptonians, such as Mala. It also affected beings with the ability to absorb or copy Superman's abilities, including Parasite and Amazo
The radiation emanated by even the smallest chunk of Kryptonite could be harnessed and used as an infinite power source. One piece was used to fuel Metallo. Another piece was used to power Lex Luthor's artificially built housing community Lexor City.
Kryptonite could be molded into different forms to suit specific purposes. Lex Luthor incorporated Kryptonite into a missile to stop Superman's rampage across Metropolis. He also managed to liquefy the substance to create an injection meant to kill both Superman and Supergirl.
In Kara's dream, she (or Galatea) is pursued by men armed with Kryptonite laser rifles. Also, Professor Hamilton constructed a surgical robot using a Kryptonite-tipped scalpel for operating on Supergirl.
- Mr. Mxyzptlk transformed himself into a missile with a Kryptonite warhead, while chasing Superman.
- Metamorpho was able to alter his chemical makeup to replicate Kryptonite's radiation.
Kryptonite being a very rare substance, there are only a few known pieces:
- The first was discovered by Lex Luthor in the collection of his Natural History Museum; after Luthor's attempt to use it to kill Superman failed, Superman hurled it into space in a lead container. A small sliver of the rock was kept by S.T.A.R. Labs.
- S.T.A.R. Labs also acquired a much larger piece, used to test the Anti-Kryptonite suit Hamilton built for Superman.
- Superman used a chunk to temporarily restrain Mala. It was unknown whether this was the chunk possessed by S.T.A.R. Labs, or another one. This chunk was sent into the Phantom Zone by Jax-Ur.
- Luthor used a chunk of Kryptonite as an infinite power source for the cyborg Metallo.
- A sizable piece fell to Earth in China and, being mistaken for jade, was carved into a statue known as "The Laughing Dragon". The statue temporarily came into possession of the Joker, but was destroyed.
- At some point, Luthor acquired another piece, which he carried with him for several years as protection against Superman, until he developed Kryptonite poisoning.
- This piece of Kryptonite eventually came into the possession of Batman, who took it from Luthor after he was exposed as a criminal and imprisoned. Batman carried it around in his utility belt as "insurance", but Amazo pulverized it by throwing it against Superman's chest. Batman eventually acquired another piece of Kryptonite that he carried around, as acknowledged by Superman.
- During the Thanagarian invasion, Thanagarian soldiers used a kryptonite energy cannon to incapacitate Superman.
- Toyman manufactured life-sized toy soldiers with Kryptonite mixed with plastic.
- Batman kept a small sliver of Kryptonite in the Batcave, to guard against the probability of Superman going rogue again. Safe from harm's way, it was kept in an underground vault behind the Batman Family display case. Exposing the vault merely needed a button pressed at the Batcomputer, but to unlock the vault itself required his entire body, as the vault's lock included two palm scanners and a full body scanner to confirm his identity.
- General Hardcastle owned a gun with a Kryptonite-tipped bullet.
- In Luthor's hideout, one of its underground tunnels was rigged with many Kryptonite pebbles.
- In Skartaris, there was a giant piece of Kryptonite called The Great Stone, which was kept in a monastery and could be used to treat maladies.
- During his final invasion of Earth, Darkseid carried a knife with a Kryptonite blade.
In popular culture, the word "kryptonite" was often used in the same sense as "Achilles Heel", to refer to a weakness which immediately disables a seemingly strong individual. Galatea once claimed that "Boredom is my Kryptonite... well, Kryptonite is my Kryptonite, but you know what I mean".