Green Kryptonite was the primary variety of Kryptonite.
It was a radioactive element composed of pieces of the exploded planet Krypton. Surviving natives of Krypton, such as Superman and Supergirl, or their descendants were weakened by exposure to Green Kryptonite. Prolonged exposure could result in fatal radiation poisoning.
Green Kryptonite radiation interferes with a Kryptonian's ability to absorb and process solar radiation which explains why Kryptonians are weakened by it even when outside in direct exposure to the light of a yellow sun. It also contaminates the energy already stored within a fully powered Kryptonian's body as prolonged exposure to and/or direct contact with kryptonite will cause a sickly green discoloration of the skin and veins will begin glowing green as the radiation continues to seep in and poison the body. The weakening affect depends on how much K-radiation the Kryptonian is exposed to. Small amounts will make a Kryptonian sick, dizzy, and feeling weaker than usual. Heavy doses will cause a severe and immediate reaction causing extreme pain, sickness, and extreme physical debilitation. While Kryptonite will quickly weaken a Kryptonian, it isn't an instant process as Batman remarked, while fighting Superman with a Green K ring, that even weakened, punching Superman was like hitting a brick wall and hitting harder would probably break his hands.
However, while the radioactive effects of kryptonite of relatively small amounts can weaken and kill a Kryptonian within minutes, they are not cumulative, so any Kryptonian safely removed from such a source can quickly recover without lasting effect. By contrast, while humans can safely handle kryptonite unshielded for short periods without ill effect from its radiation, long-term continual exposure will produce serious health problems such as cancer.
- Kryptonite was first created for an unpublished 1940 comic book story, in which it was called "K-Metal". Kryptonite made its actual debut in a 1943 episode of the The Adventures of Superman radio series. In 1949's Superman #61, Kryptonite made its DC Comics debut, colored red but with the properties commonly associated with Green K and not the unpredictable effects of Red Kryptonite. Reprints of this story have recolored the Kryptonian mineral as green. Green K in its commonly recognized form finally made its comics debut in 1951's Action Comics #161.