Kryptonite

1) (Universe: DC (Pre-Crisis)) Element created from the substance of the planet Krypton (2) after its destruction. A radioactive element, it has a half-life varying between several years to a century long. There are a number of varieties and isotopes of kryptonite, which (unless otherwise noted) only affect Kryptonian lifeforms. These are:

http://www.dracandros.com/Jebgarg/Ni...ryptonites.jpg

Green Kryptonite, which weakens and eventually kills only super-powered Kryptonians (as in, dwelling under a yellow sun). This is the most common form.
Anti-Kryptonite, which weakens and eventually kills non-super-powered Kryptonians (dwelling under a red sun). Argo City (1) was built on Anti-Kryptonite, which was responsible for destroying most of its population.
X-Kryptonite was an isotope of Green Kryptonite accidentally created by Supergirl (1) when she was trying to find a cure for Kryptonite poisoning. X-Kryptonite gives temporary Kryptonian super-powers to Earth life forms.
Slow-Kryptonite was created in an unrepeatable experiment on Earth, and affected humans the same way Green K affects super-powered Kryptonians.
Red Kryptonite was normal Green Kryptonite which was altered by a weird space cloud. A piece of Red Kryptonite has the same, unpredictable effect on every Kryptonian who encounters it, generally in the form of physical mutations and/or mental transformations. Its effects last between 24 and 48 hours, and after a Kryptonian is affected by an individual piece, they are thereafter immune to that piece. However, since Red K is so bizarre and random in nature, there are certainly exceptions.
White Kryptonite emits radiation that can kill plant life of any world, not merely that of Krypton.
Blue Kryptonite was created from Green K by the same process that created Bizarro (1), and only affects Bizarro-Supermen, working in the same way on them as Green K does on Kryptonians. (Note that they were grateful to Superman for creating something that can destroy them.)
Gold Kryptonite permanently robs a Kryptonian of their super-powers. It is believed to be created by subjecting the Red or Green varieties to intense nuclear radiation.
Jewel Kryptonite was deliberately created by the time-travelling Phantom Zone Criminal Jax-Ur, who prepared crystals from the Jewel Mountains of Krypton to change into Jewel K upon Krypton's end. It was used by the Phantom Zone Criminals to give them psychic abilities projecting outside the Zone, but Superman since destroyed most, if not all, of it.
Silver Kryptonite was a false form concocted by Jimmy Olsen (1) as a joke on the occasion of Superman's birthday.

All Kryptonite eventually decays to iron. Unlike most Kryptonian materials, Kryptonite does not become invulnerable under a yellow sun- but it does become friction-proof, thus able to withstand atmospheric entry. (These myriad varieties of Kryptonite ceased to exist after the Crisis on Infinite Earths, though the element can still be found in certain hypertimelines. In fact, samples of Green, Blue, Gold, and Red K can be found in the Planet Krypton restaurant in Metropolis.)
2) (Universe: DC (Post-Crisis)) Element created from the substance of Krypton (3) after its destruction. This element is radioactive, and can weaken and kill a Kryptonian in a relatively short amount of time. However, the radiation also affects non-Kryptonian life as well, slowly causing the effects of radiation poisoning, as Lex Luthor (4) was unfortunate enough to find out. A Red Kryptonite variety was created by Mr. Mxyzptlk (2), but it was only really a device by which to grant Lex Luthor a wish. Another form of Red Kryptonite- an altered version of normal Kryptonite conceived by Batman (4) as a possible last-ditch weapon against a dangerous Superman- could cause Kryptonians intense, incapacitating agony.
3) (Universe: Superman (Films)) Identical to the Green variety of (1). A synthetic version was created by August "Gus" Gorman at the behest of Ross Webster, but the 0.57% of the element which could not be replicated by Earth materials were replaced- at Gorman's suggestion- with tar. The result was a Kryptonite that amplified Superman (4)'s dark side, forcing his good and evil sides to battle for dominance- a battle which the good side won.
4) (Universe: Lois & Clark) (1)'s Green and Red varieties are the Kryptonites of this universe.
5) (Universe: DC Animated) Identical to (1)'s Green variety. It also affects natives of Argo (2), Krypton (6)'s sister planet. (In the Superman Adventures comic, Kryptonite does not affect Argo natives. This was contradicted by the televised episodes "Legacy" Parts I & II, wherein Supergirl (3) (an Argoan) is felled by a Kryptonite missile.)
6) (Universe: Smallville) A sort of combination of (1)'s Green and X varieties. While Clark Kent (7) is affected by Green Kryptonite as described in (1), it can also cause mutations in normal humans like X, giving them super-powers that often reflect their personalities or desires. Red Kryptonite also exists here, which can negatively affect Kryptonians' minds.

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wow. that's alot of info.

To paraphrase Jack Handey:

"You know what I want? I want a Kryptonite cross. That way I can keep away vampires and Superman."

Kryptonite tastes funny