Despicable Me 3
The diamond was being transported on a large, guarded ship when Bratt made his attempt to steal it. Not long after he got his hands on it, Gru and Lucy arrived at the sense to stop the theft and to bring him in. Despite the diamond being secured, the Anti-Villain League's new director, Valerie Da Vinci fired Gru and Lucy for failing to catch Bratt, again.
The Dumont Diamond was later placed in a museum in France as one of the exhibits; where a plump diamond expert (known as Monsieur Pompo) came to make sure that the gem wasn't switched for a fake in the in tempted theft. The diamond expert later turned out to be Bratt in disguise, he knocks the guards out by throwing knock-out dart at them and then takes the diamond back to his lair. Bratt later tells his sidekick, Clive that he plans to use the Dumont to power the lazar on his giant robot replica of himself; much like the Evil Bratt episode that he had drew his plan from.
Gru, with the help of his twin brother, Dru later stole the diamond from within Bratt's lair as his last hast as a villain. Dru believed that they were stealing it so they could become the richest villains in the world, while Gru wanted to use it to get his and Lucy's jobs in the AVL back by giving them the diamond. As the Gru family packed up and were about to leave Freedonia, Bratt disguised himself as Lucy as he steals the diamond back and kidnaps Margo, Edith and Agnes. He then goes along with his plan as he takes his revenge on Hollywood.
What happened to the diamond after the giant robot was destroyed is unknown.
- In the junior novelization and the original script, the diamond is called the Dupont diamond while in the DVD/Blu-Ray subtitles, it's called "Dumont". This led to a lot of confusion. Most fans still aren't entirely sure which spelling is correct.