Kryptonite is a material from of ore form a radioactive element from the planet Krypton. It is the ultimate natural weakness of Superman and most other Kryptonians.
History
Initially, it was Lex Luthor who conducted research about planet Krypton and deduces that a meteorite found in Addis Ababa, Ethiopia is actually a radioactive piece of the perished planet. Figuring out that Superman would be vulnerable to its radioactivity, Luthor and his cronies, Eve Teschmacher and Otis, murder a guard and steal it from a museum under the cover of night. After co-opting and launching two missiles for opposite ends of the United States, Luthor places the Kryptonite on a chain around Superman's neck and drops him into a swimming pool. When Teschmacher learns that one of the missiles is headed for Hackensack, New Jersey (where her mother lives), she rescues Superman from drowning, kisses him and removes the Kryptonite, after which his strength and powers quickly return.
Years later, billionaire Ross Webster orders the creation of synthetic Kryptonite after remembering a Daily Planet story about the last original chunk disappearing years earlier after falling to Earth. After scanning the coordinates of Krypton's former location via satellite, computer programmer Gus Gorman discovered Kryptonite's composition. The substitution of tar (which Gorman used after glancing at a pack of cigarettes) for the crucial, but unknown, component resulted in the synthetic Kryptonite—like black Kryptonite in the comics—turning Superman evil and eventually splitting him into two beings. Retaining Superman's remaining good qualities, Clark Kent fights and defeats the dark Bizarro, who vanishes. Later in the film, Gorman's creation, the Ultimate Computer, severely weakens Superman with a Kryptonite ray before Gorman has a change of heart and attacks his own machine.
Composition
Gus Gorman determined that Kryptonite was "an intense heat fusion" of: 15.08% plutonium, 18.06% tantalum, 27.71% xenon, 24.02% promethium, 10.62% dialium (which is a plant genus, not an element), 3.94% mercury, with the last 0.57% being an unknown element not found on the known periodic table.