Green Kryptonite is the most abundant and most frequently used form of kryptonite. But it wasn't always been part of the Superman universe, and it wasn't always green. An unpublished Superman story from the 1940s featured a precursor to kryptonite, a substance called K-metal. The deadly element itself made its debut in the "Superman" radio series -- not the comic book -- in 1943. Its original purpose was to give voice actor Bud Collyer, who played the role of Superman, a vacation. With Superman incapacitated by kryptonite, another voice actor could supply his incomprehensible moans, filling the role until Collyer returned.
Kryptonite made another radio appearance in 1945 and appeared in a movie serial in 1948. But it didn't find a place in the comics until "Superman" #61 in 1949, more than 10 years after Superman's debut in "Action Comics" #1. Radio scripts and comic book art portrayed the substance as red, grey, green and metallic, but eventually the writers settled on green as the color of kryptonite.
In addition to "plain" Green Kryptonite, multiple varieties have appeared on the scene through the years. These varieties are different isotopes of the same element, and they can come in different grades, or strengths. The different isotopes have distinctly different effects on Superman and other life forms, but these effects most likely all stem from the disruption of cells. Some varieties of kryptonite appeared in only one comic book issue or story arc. For example:
- Jewel Kryptonite, or Kryptonite 6, enhanced the powers of Kryptonians who had been sentenced to live in the Phantom Zone. It appeared in "Action Comics" #310.
- X-Kryptonite appeared in "Action Comics" #261. It caused a cat named Streaky to develop superpowers.
- Kryptonite Plus was an extra-potent variety that appeared in two story arcs.