A jewelry-heist thriller of a lawsuit is unfolding in a Manhattan court involving an extremely rare pink diamond, its $40 million sale to an anonymous buyer and an Italian heir’s claim that his two-timing stepbrother stole the beautiful bauble from his dead mother.

Manhattan-based Italian citizen Amedeo Angiolillo, the sole surviving child of the late Senator Renato Angiolillo, is suing Christie’s auction house and the “John or Jane Doe” who purchased the famed Princie Diamond in a record-breaking sale in April 2013.

Amedeo’s father, “who had a passion for the collection of fine jewels,” bought the 300-year-old, 34.65-carat Indian diamond from Van Cleef & Arpels in 1961, his Manhattan Supreme Court suit says.

The Il Tempo daily newspaper founder let his second wife, Maria Girani, wear the illustrious diamond, named after Indian Prince Osman Ali Khan, according to court papers.

But Girano “did not have unfettered access to” her husband’s gems, Amedeo sniffs in the suit.

When Renato died in 1973, Girani made an arrangement with Amedeo to wear the jewel until her death, the suit says.

“Girani had a good relationship with her husband’s children and grandchildren, confirming on several occasions that the gems in her possession, including the Princie Diamond, belonged to them by inheritance, and that they would be returned to them after her death,” Amedeo says in the filing.

But when she passed in 2009 without a will, the bauble went missing. Her own son, Marco Oreste Bianchi Milella, claimed he had never seen the Princie Diamond or other jewelry among his mother’s possessions.

“Those assets were not found at the residence of my mother,” Milella stated through his attorney in 2012.

But then a year later, Amedeo learned Christie’s was about to auction off a rare pink diamond that he was nearly certain had belonged to his late father.

He learned from Christie’s that his stepbrother, a one-time roommate and friend, told the auction house he was the “100 percent beneficial owner of the Princie Diamond.”

Christie’s also revealed that “an anonymous consignor purchased the Princie Diamond through a broker for over $20 million — about 50 percent of its auction price … a few years later,” the filing says.

Police found other gems belonging to Renato at Milella’s Monte Carlo home, including a ruby red necklace gifted to his girlfriend, according to court papers.

Yet Christie’s refused to stop the sale despite a pending criminal case in the Court of Campobasso, Italy, against Milella because the auction house said it had “valid title” from a Swiss corporation, the suit says.

“New York law provides that once there is a ‘thief,’ an involuntary transfer, or if the property is lost in the chain of title, no one can take good title,” according to court papers.

Amedeo says he has an insurance certificate to prove the diamond belongs to him and his nieces and nephews, who inherited Renato’s estate.

He wants the diamond back plus additional damages.

A rep for Christie’s did not immediately comment.