"To steal the diamond" means to secretly take someone else's valuable diamond without their knowledge or permission. It refers to an act of theft or stealing. Full definition
«Sara Taylor (Monaghan), a photojournalist blinded in Iraq, is forced to play a deadly game of cat and mouse with the sadistic criminal Hollander (Keaton)-- a man who will stop at nothing to get a fortune of stolen diamonds hidden in Sara's penthouse apartment. (bloody-disgusting.com)
Leaden with implausibility (You'll wonder throughout how Nick prove his innocence by subversively trying to steal the diamond from the man who framed him), its overreaching script by Pablo Fenjves (The Affair, The Devil's Child), Man on a Ledge traverses through clichés and hard - to - swallow machinations in order to keep the audience on its toes. (qwipster.net)
Also, you never find out in the 102 minute running time why exactly Ed Harris set up Sam Worthington for stealing the diamond. (weliveentertainment.com)