Kryptonite is a material from the Superman fictional universe, specifically the ore form of a radioactive element from Superman's home planet of Krypton. First mentioned in The Adventures of Superman radio show in June 1943, the material has been featured in a variety of forms and colours (each with its own effect) in DC Comics publications and other media, including feature films, television series, and novelty items such as toys and trading card sets. The classic form of kryptonite comes in glowing GREEN!
The established premise is that Superman and other Kryptonian characters are susceptible to its radiation, which created usage of the term in popular culture as a reference to an individual's perceived weakness, irrespective of its nature, similar to the term "Achilles' heel."
Seriously now, my "kryptonite" here is just broken green glass...
This post is part of the Friday Greens meme.