Kryptonite is a rare crystal believed to be piece of the late planet Krypton. It is element 126 (a fictional stable transuranic element) created when the radiation of Krypton's red giant sun converted the crystalline crust of the planet into kryptonite when the sun went nova. An unknown amount of the substance arrived on Earth along with the ship that brought baby Kal-El to Earth - presumably caught in the hyper-drive wake of the ship. Kryptonite has a crystalline structure similar to quartz and is fluorescent with a green glow.
Green kryptonite is known to have extremely deleterious effects on Kryptonians. Exposure causes dizziness and weakness, muscle and joint pain, but passes quickly when exposure ends. Extended exposure can cause death.
Red kryptonite was not shown in the films but a reasonable assumption can be made that since green kryptonite negatively effects the Kryptonian body, red kryptonite would effect the Kryptonian mind.. (Or you could use the comic book version that Red K causes temporary bizarre effects that can be either mental or physical.)
Placard from the Metropolis Museum of Natural History (Superman Returns)
Given that kryptonite has been established to be a transuranic element and the placard does not list any 'unknown' elements as part of the meteorite the placard describes, it can only be assumed that the sample used for the analysis was from the surface of the meteorite and did not include any of the crystalline interior mass. It can also be assumed that the sodium lithium boron silicate hydroxide with fluorine is a sample of Krypton's mantle. (Sodium lithium boron silicate hydroxide is a gray powder.)
However, in Superman III, kryptonite is said to be:
|Kryptonite ... An Intense Heat Fusion Of:|
When this compound is created artificially with 'tar' replacing the unknown missing element, this has profound deleterious effects on Superman, creating depression, and violent anti-social tendencies.
Daily Planet Guide to Metropolis, from WEG
Superman: The Ultimate Guide to the Man of Steel (Hardcover) by Scott Beatty (Author)
Wikipedia Entry for Kryptonite