In 2010, an accident occurs at a mine operated by Ethiopian Jewish workers. During the commotion, two miners retrieve a rare and valuable black opal, leaving it embedded in a piece of rock.
In 2012, Howard Ratner runs a jewelry store in New York's Diamond District while struggling to pay back gambling debts, particularly $100,000 to Arno, a loanshark who happens to be his brother-in-law. His domestic life is split between his employee and mistress Julia, and his estranged wife Dinah, who has agreed to divorce him after Passover. One day, Howard receives the rock containing the opal, which he plans to auction off at an estimated price of $1,000,000. Basketball player Kevin Garnett visits the shop and becomes obsessed with the opal, insisting on holding onto it for good luck at his game that night. Howard reluctantly agrees, but requests to keep his Celtics ring as collateral. Garnett agrees, and shortly after Howard pawns it and places a bet on Garnett's game that night.
Howard wins the bet but is later ambushed at his daughter's school play by Arno and his bodyguards, Phil and Nico. He is shocked to learn that Arno has placed a stop on the bet, as it was made with money owed to him. Phil and Nico then strip Howard naked and lock him in the trunk of his own car, forcing him to call Dinah for help. Howard hurries to retrieve the opal at a party hosted by The Weeknd and ends up getting into a fight with the singer after discovering him and Julia together in a bathroom. Howard chastises Julia and throws her out of his apartment despite her insistence that she remained faithful to him.
Garnett returns the opal and offers to purchase it, but Howard refuses given the substantially larger appraisal. Just before the auction starts, Howard is devastated to discover the opal has been appraised for significantly lower than his estimate. Desperate, he convinces his father in law, Gooey, to bid on the gem to drive up the price, but this backfires when Garnett bows out early and Gooey wins. Arno, Phil, and Nico confront him outside the auction and proceed to punch him in the face and throw him into a fountain. Howard returns to his shop battered and emotionally broken, where Julia comforts him and the two repair their relationship. Immediately afterward Howard is informed that Garnett still wishes to purchase the opal.
The deal is completed, but instead of immediately paying off his debts, Howard becomes obsessed with betting on Garnett's connection with the opal. He gives the money to Julia, instructing her to put it all on Garnett's game that night. She boards a helicopter to Mohegan Sun, barely escaping Arno, Phil and Nico, who have come to collect. They become furious when they learn of the bet, but Howard locks them in the mantrap of the store as they try to leave and pursue Julia. He then frantically watches the game with his money on the line, with Arno and his muscle watching alongside. Ultimately Howard's bet pays off, winning him over $1.2 million.
When a celebratory Howard releases the trio, a fed-up Phil immediately shoots him in the face. Horrified, Arno protests and tries to escape, but Phil shoots him in the head as well. An overjoyed Julia leaves Mohegan Sun with the winnings on her way back to Howard, unaware that he has been killed, while Phil and Nico loot the store as the deceased Howard lies bleeding on the floor.
