This is the story of one of those diamonds so exquisite it was given a cute little name: the Princie.

Yet despite its diminutive moniker, the Princie is a big stone — 34.65 carats — that sparkles in the cheerful color of a flamingo and is valued at $40 million. It was last known to be packed away in a storage facility in Switzerland, where it was sent by a member of the Qatari royal family after he bought it.

But is that where it should be?

That question is the subject of a trial about to begin this week in New York Supreme Court where an Italian family has accused Christie’s, the auction house, of selling the diamond despite accusations that it had been stolen.

The descendants of a once powerful Italian senator say the diamond is rightfully theirs, and that their stepbrother absconded with it after his mother died. But the stepbrother has insisted the diamond was his to sell. Christie’s says the family members have no proof the diamond belongs to them and that, regardless, its client, who bought it from the stepbrother, had every right to sell the stone.

It is a case sprinkled with royalty and rich people, the nuances of Italian law and with questions about the responsibilities of auction houses when there is a dispute about who owns a sale item.

The diamond, cut from the Golconda mines in India, made its first recorded appearance in the 1700s as part of the collection of the Nizam of Hyderabad, an Indian monarch. The plaintiffs say the diamond, about the size of a Cerignola olive, was bought by Senator Renato Angiolillo at Van Cleef & Arpels in 1960, the same year he married his second wife, Maria Girani Angiolillo. It had been named “Princie” in honor of the 14-year-old Prince of Baroda, a former state of India, who came to a party that year at the Van Cleef & Arpels store in Paris, along with his mother.

Mr. Angiolillo died in 1973, and here, things get complicated. Under Italian law at the time, as court documents explain, all of his possessions should have gone to his children, not his spouse, unless they were explicitly left to her. His will said his wife should keep their home near the Spanish Steps in Rome and its lavish furnishings. But nothing else was mentioned. So the lawsuit argues that the rest of the estate, including the diamond, belongs to his descendants — Mr. Angiolillo’s surviving son and four grandchildren are the plaintiffs in this case.

But the auction house and its co-defendants said that the diamond, set in a ring, was a gift to Ms. Angiolillo, and so was owned by her, not her husband, when he died. And even if the transfer of ownership between them was not official, the defendants argue, the way she kept control of the ring in the decades that followed his death made it legally hers.

No one disputes that Ms. Angiolillo held on to the diamond for more than 35 years after she became his widow, but Mr. Angiolillo’s descendants say it should have been turned over to them after she died as part of their inheritance.

Instead the whereabouts of the diamond became something of a mystery. Amedeo Angiolillo — the senator’s remaining son — tried to contact his stepbrother, Marco Milella, to ask for the diamond back. He had no luck. Eventually, Mr. Milella responded through a lawyer who said, in essence: Diamond? What diamond?

Read the complete original article on nytimes.com