Spoilers ahead for Superman (2025)! In the film, Lex Luthor captures Superman and throws him into a pocket universe prison, where he's placed in a cell with the metahuman Metamorpho (Rex Mason). Luthor then coerces Metamorpho, who can transmute his body into any chemical element or compound, to continuously turn his hand into synthetic Kryptonite. The radiation keeps Superman powerless and in constant agony.
The movie treats it as a given that Lex already knew Kryptonite's composition and its effects on Kryptonians before the story even begins, but it never explains how he came by that knowledge.
Knowing that Kryptonite exists is one thing; knowing it's specifically Superman's weakness is another. So how did Lex Luthor learn that Kryptonite was Superman's weakness?
Luthor discovered Kryptonite's effect on Superman by accident, while using it as a power source for Metallo
The prequel junior novel Superman: Welcome to Metropolis reveals that LuthorCorp possessed a glowing green ore, implied to be Kryptonite. Suspecting its potential as a weapon, Lex Luthor used it as an energy source to power the cyborg villain Metallo (John Corben).
When Metallo clashed with Superman, the Kryptonite inside him severely weakened Superman. Superman ultimately defeated Metallo with help from the Justice Gang, but the encounter gave Luthor his first real evidence of Superman's vulnerability to the rock.
In Chapter 17, Lex initially uses the mineral purely for its immense power output, with no knowledge of its harmful effects on Superman:
“An ore—or mineral—I’m not a geologist,” Luthor answered. “Extremely rare. Previously unknown to science. Unlike anything ever seen on Earth before. LuthorCorp has mining interests all over the world, and when this new ore was discovered, it was immediately brought here for our experts to examine.”
“How is a rock supposed to make me stronger?” Metallo asked skeptically.
“The ore is a source of tremendous power we haven’t known before,” Luthor explained. “It doesn’t have to be burned or combusted like a traditional petrochemical fuel. It emits power that can easily be channeled to whatever purpose we choose.” Luthor smiled. “I choose to make you more powerful than Superman.”
Moore hesitated to speak up, but he felt he had to. “We’re still in the very early stages of studying the mineral. We’re not sure what other effects it might—”
The weakening effect on Superman first shows up in Chapter 20, during the fight between Superman, the Justice Gang, and Metallo. After the fight, Superman notes:
“Somehow close proximity to Metallo sapped my powers.”
Mister Terrific furrowed his brow. “That’s strange—it didn’t happen when you fought him before, did it?”
Superman shook his head. “No. Apparently something’s changed. The only outward sign I noticed was—”
“His robot eye changed from red to green,” Hawkgirl said.
In Chapter 21, while reviewing the battle footage, Luthor calls the outcome "instructive" and says he was "particularly intrigued" by it, indicating he is only now learning about Superman's Kryptonite vulnerability rather than having known about it all along.
“Sorry Metallo lost the big fight to Superman,” Otis said, consoling his boss. “You watched the whole thing with Metallo’s camera?”
“Of course. Yes, the cyborg’s defeat was disappointing,” Luthor admitted, “but instructive. I was particularly intrigued by Superman’s reaction to Metallo’s new power source.”
“That green stuff?” Otis asked. “The rock?”
“Precisely,” Luthor said, opening the metal box on his desk. A green light shone from within. “The rock.”
