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.
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- Kryptonite was first created for an unpublished 1940 comic book story, in which it was called "K-Metal".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.