Thieves have stolen two diamonds worth about 11.5m euros (£7.82m) from a trade fair at one of Paris' top museums.
The gems were taken from a display case at an antique and jewellery dealers' show at the Louvre Museum complex.
The thieves pounced when employees left the Chopard booth for about 15 minutes, police were quoted as saying.
The BBC's Allan Little, in Paris, says the theft happened in broad daylight in one of the most public places in the city, and that there was no violence.
It appears that the display case was not alarmed, although there are many security cameras in the complex, our correspondent says.
Police described the apparent lack of security as "rare".
The biggest diamond stolen was a 47-carat jewel worth an estimated six million euros. The other stone taken was about 30 carats, worth 5.5 million euros.
Security measures for the exhibition were not connected to the security for the Louvre Museum itself, which houses some of the world's priceless works of art, such as Leonardo da Vinci's Mona Lisa.
The Antique Dealers Biennale at the Carrousel du Louvre, which began on 15 September and runs until 28 September, brings together more than 100 antiques and jewellery dealers.
Also on display at the Biennale was the 128.48-carat "Star of the South" - one of the world's most famous diamonds known for its pinkish-brown colour.
Cartier had put the precious gem up for sale.