“Smallville” Season 7, Episode 9: “Gemini”

Here is the official description from the CW:

CHLOE REVEALS TO JIMMY THAT SHE IS A METEOR FREAK – Clark (Tom Welling) returns to Smallville from the Fortress and tells Lana (Kristin Kreuk) that Kara (Laura Vandervoort) is gone. Clark tells Lana they should team up to bring Lex (Michael Rosenbaum) down, so she gives him all the information she’s gathered so far, including a report on a man named Adrian (guest star Tim Guinee), who was infected with an alien substance.
Adrian secretly plants a bomb on Chloe (Allison Mack) and tells Lois (Erica Durance) if she doesn’t get Lex to admit the truth about his experiments with 33.1, he will detonate the bomb. Chloe, thinking she is about to die, confesses to Jimmy (Aaron Ashmore) that she is a meteor freak. John Glover also stars. Whitney Ransick directed the episode written by Caroline Dries.