This article contains spoilers from the most recent DCEU film. You have been warned.
The Bertinelli Diamond was an extremely valuable diamond that was once owned by the Bertinelli Crime Family.
The Bertinelli Diamond was the key to the treasure of this powerful crime family, but the entire family, save the Huntress, was killed. Many years later, the Black Mask (Roman Sionis), himself a powerful member within the underworld of Gotham City, sends his top lieutenant, Victor Zsasz, to take possession of the diamond in order to cement his place at the top of Gotham City' crime world. Zsasz succeedds in securing the diamond, but to his shock ends up losing it to a young pickpocket and thief named Cassandra Cain. By the time he realizes the theft, it is too late, as Cain has already been picked up by the Gotham City Police Department, being driven out of his reach. While on her way to jail, Cain swallows the diamond.
Meanwhile, the Black Mask learns of Harley Quinn's emancipation from the Joker and that it has become essentially open season on those with a grievance against her. He ties her up and threatens to kill her in the most painful way, but in the process she overhears him complaining about the loss of his diamond. She offers her skills as a finder of lost things, telling him that he has nothing to lose. He decides that it is worth a shot and releases her, but also puts out a general $100,000 reward for anyone who can capture Cain alive.
Following a big battle at the Gotham City Police Department, Quinn manages to become the one to secure Cain. She decides, however, that what the Black Mask has offered her isn't enough and calls him back, making a different deal - protection against the others who have vengeance against her. Meanwhile, she begins dosing Cain with laxatives, burritos and others aids to try to get the diamond out of her before less savory individuals cut it out of her. She takes Cain to an abandoned amusement park, where she plans to turn her over to Victor Zsasz. Ultimately, however, she bonds with the group who will become the Birds of Prey - Renee Montoya, Black Canary and the Huntress, to take down the Black Mask. The diamond is safely recovered from Cassandra Cain. It is then used by the Huntress, whose birth name is Helena Bertinelli and thus the diamond's rightful owner, to unlock the fortune and fund the Birds of Prey. The diamond is further used by Harley and Cain when they sell it to a pawnshop and use the money to start their own business.
- Key: This diamond was the key to unlocking the fortune of the Bertinelli Crime Family.