Fawaz Gruosi, the founder of the Swiss firm, said the necklace was part of the company’s 20th anniversary collection paraded by 20 models at the glitzy event at the Hotel Du Cap-Eden-Roc in Antibes outside Cannes on Tuesday night. Sharon Stone and Paris Hilton were among the guests.

Gruosi said 80 bodyguards, local police, hotel security, and De Grisogono staff had been on duty but when the final check was conducted at the end of the night the long-stranded diamond necklace was missing.

“We don’t know exactly what happened … it was one of the most beautiful items we had,” Gruosi told Reuters Television.

“The police are trying to figure out what happened.”

No one from the local police was immediately available to comment.

Another Swiss jeweller, Chopard, had gems worth $1.4 million stolen in the first week of the 12-day festival on the glamorous French Riviera, which attracts thousands of actors, filmmakers and journalists.

Police said the Chopard jewellery, intended to adorn movie stars at the festival, had been in the safe of a room at the Suite Novotel four-star hotel in central Cannes, which had been rented by an employee of the jeweller.

The entire safe was removed from the wall and taken from the room during the night of May 16. Someone entered without either forcing the door or using the magnetic key card, a police source said.

A spokesman for Chopard, a sponsor of the Cannes festival, later played down the report, saying the value had been exaggerated and the items were not for actresses to use.

Jewellers and fashion houses use the world’s largest film festival at Cannes as a promotional showcase, lending gowns and accessories to celebrities who are photographed on the famed red carpet and at parties along the palm-lined Croisette waterfront. DM

Photo: Actor Robert Redford (R) and his wife Sibylle (2ndR) pose with director J.C. Chandor (4thR), producer Neal Dodson (3rdL) and his wife actress Ashley Williams (4thL), and producer Anna Gerb (2ndL) as they arrive for the screening of the film “All is Lost” during the 66th Cannes Film Festival in Cannes May 22, 2013. REUTERS/Yves Herman