Kryptonite is a fictional material that appears primarily in Superman stories published by DC Comics. 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. Kryptonite radiation can emit through any element except lead. Thus, Superman has a special lead suit to protect himself from the radiation. There are other varieties of kryptonite, such as red and gold kryptonite, which have different but still generally negative effects. Due to Superman's popularity, kryptonite has become a byword for an extraordinary exploitable weakness, synonymous with "Achilles' heel". Batman, Lex Luthor, Metallo, and Titano are four notable characters often presented as using kryptonite — the first carrying the substance as a last-ditch method to stop his ally Superman if he is subject to mind control or otherwise compromised, the next two using the mineral to ward off Superman or incorporating it into weapons, and the fourth being able to project rays of kryptonite radiation from his eyes after being altered by simultaneous exposure to kryptonite and uranium.