A Fish Called Wanda

London gangster George Thomason and his right-hand man, Ken Pile, an animal lover with a stutter, plan a jewel heist. They bring in two Americans: con artist Wanda Gershwitz and weapons expert Otto West, a mean-spirited anglophobe. Wanda and Otto are lovers, but they hide this from George and Ken, pretending to be siblings, so Wanda can work her charms on them. The heist is successful, and the gang escapes with a large sum in diamonds. They hide them in a safe in an old workshop. Soon after, Wanda and Otto betray George to the police and he is arrested. They return to collect the diamonds, with Wanda planning to double-cross Otto, as well, but find that George has moved them. In Ken's fish tank, Wanda discovers the key to the safe deposit box containing the diamonds and hides it in her pendant.
Wanda decides to seduce George's barrister, Archie Leach, so he can persuade George to plead guilty and give up the location of the diamonds. Archie is in a loveless marriage and quickly falls for Wanda; Otto is jealous, and his interference causes Wanda and Archie's liaisons to go disastrously wrong. Wanda accidentally leaves her pendant at Archie's house, which Archie's wife, Wendy, mistakes for a gift for her. At Wanda's insistence, Archie recovers the pendant by staging a burglary. Eventually, Archie, feeling guilty, ends the affair.
George asks Ken to kill Mrs Coady, the Crown's only eyewitness. Though Ken accidentally kills her three dogs, causing him great distress, he is successful when their death gives her a fatal heart attack. Wanda and Otto want George to remain in jail, but with no witness, he now seems set to get off. At his trial, defence witness Wanda unexpectedly gives evidence against him. When Archie, stunned, flubs his cross-examination and inadvertently calls her "darling", Wendy realises that Archie has had an affair and decides to divorce him. Otto tries to force Ken to reveal the location of the diamonds by eating his pet fish, leaving Ken's favourite, named Wanda, until last. Ken reveals that the diamonds are at a hotel near Heathrow Airport.
With his career and marriage over, Archie resolves to cut his losses, steal the loot himself, and flee to South America. Promised less jail time, George tells Archie that Ken knows where the diamonds are. Archie sees Wanda fleeing the courthouse, pulls her into his car, and races to Ken's flat. As Archie runs into the building, Otto steals Archie's car, taking Wanda with him. Ken and Archie give chase. Otto and Wanda recover the diamonds, but Wanda double-crosses him and leaves him unconscious in a broom cupboard. Recovering, Otto shoots his way out of the cupboard and is confronted by Archie. Otto is about to kill him, but Archie stalls him by taunting Otto about American failures such as the Vietnam War. Ken arrives driving a steamroller, seeking vengeance for the fish; Otto, who has stepped in wet concrete and cannot move, is run over but survives. Archie and Wanda board the plane to start a new life in South America. Otto, clinging to the window outside, curses them until he is blown off during takeoff.

Question: What is the gang planning to steal?
Answer: Diamonds