The Kryptonite Ring is an ornamental ring affixed with a gem made out of Kryptonite. The ring was created and used by various individuals to ensure protection or to be used as a weapon against Kryptonians, most notable of which is the superhero Superman.
After discovering that Kryptonite could harm Superman, Lex Luthor used a sample to fashion a ring for himself. He wore the ring at all times to keep Superman away from him. However, long-term exposure to kryptonite radiation from the ring gave Luthor cancer, leading to the amputation of his hand and later to his apparent death (although he was able to have his brain transplanted into a cloned body).
After losing his hand, Luthor stopped wearing the ring. The ring than came into the possession of Amanda; an ex-LexCorp scientist who had discovered Superman's identity. She intended to prove that Clark Kent was Superman by confronting Clark while wearing the ring. While she was successful, she immediately became a victim of a mugging and was killed for the contents of her purse.
Droping the ring during her death, the ring than fell into the hands of a wandering vagrant who made his way to Gotham City before dying. After discovering the body, the ring fell into the possession of Batman who hands it over to Superman following a case together.
Superman then entrusts the ring to Batman, stating that he was the only person he could trust with the ability to kill him if necessary; Batman subsequently kept the ring available whenever circumstances required him and Superman to work together in case Superman ever loses control of his powers or if he falls under the control of an evil influence.
During the Infinite Crisis, Batman attempts to use the ring on Kal-L only for the hero to destroy the ring as it represented "the paranoia and mistrust that will destroy your world if you let it."
After a long-term mission by Batman and Superman to destroy all the kryptonite on Earth, Superman saves a piece and gives it to Batman for emergencies to replace the destroyed ring.