The Bertinelli Diamond was an extremely valuable diamond that was once owned by the Bertinelli crime family.
Owned by the Bertinellis
The Bertinelli Diamond was the key to the treasure of this powerful crime family, but the entire family, except Helena Bertinelli, was killed by the Galante Crime Family years ago under the orders of Roman Sionis.
Manhunt for the Diamond
Many years later, Black Mask became a powerful member within the criminal underworld of Gotham City and sent his top lieutenant, Victor Zsasz, to take possession of the diamond in order to cement his place at the top of Gotham's crime world. Zsasz succeeded in securing the diamond, but to his shock, he ended up losing it to a young pickpocket named Cassandra Cain. By the time he realized the theft, it was too late, as Cain has already been picked up by the Gotham City Police Department, being driven out of his reach. On her way to jail, Cain swallowed the diamond.
Meanwhile, the Black Mask learned of Harley Quinn's emancipation from the Joker and that it has become essentially open season on those with grievances against her. He tied her up and threatened to kill her in the most painful way, but in the process, she overheard him complaining about losing the diamond. She offered her skills to find lost things, telling him that he has nothing to lose. He decided that it is worth a shot and releases her, but also puts out a $100,000 reward for anyone who can capture Cain alive.
Following a big battle at the Gotham City Police Department, Quinn managed to secure Cain. However, she decided that what the Black Mask offered her wasn't enough and called him back, making a different deal — protection against those who want vengeance against her. Meanwhile, she begins dosing Cain with laxatives, burritos, and other 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 Zsasz. Ultimately, however, she bonded with the group who would become the Birds of Prey — Renee Montoya, Black Canary and the Huntress — to take down the Black Mask.
After the death of Black Mask, the diamond was safely recovered from Cain, and 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 was further used by Harley and Cain as they sold it to a pawnshop to start their own business.
- Cassandra Cain's character poster for Birds of Prey depicts her surrounded by a number of diamonds, one of which, stashed in a jacket pocket, presumably represents the Bertinelli diamond.
- Hodson, Christina (writer) & Yan, Cathy (director) (February 7, 2020). Birds of Prey (And the Fantabulous Emancipation of One Harley Quinn).