In its best-known form, it is a green, crystalline material originating from Superman's home world of Krypton that emits a unique, poisonous radiation that can weaken and even kill Kryptonians.

An unpublished 1940 story titled "The K-Metal from Krypton", written by Superman creator Jerry Siegel, featured a prototype of kryptonite.

[3] An apocryphal story claims that kryptonite was introduced to give Superman's voice actor Bud Collyer time off.

In the comics' Silver Age, which places Krypton in a distant solar system, much of the kryptonite that came to Earth was transported by the same "space warp" that baby Kal-El's rocket traversed.

[7] Editor Dorothy Woolfolk stated in an interview with Florida Today in August 1993 that she felt Superman's invulnerability was "boring".