Coloured synthetic diamonds on display at De Beers' International Institute of Diamond Grading and Research in Maidenhead. File photo Reuters

Japanese police are investigating a 200 million yen ($1.84 million) diamond allegedly stolen from an international jewellery exhibit near Tokyo.

The 50-carat diamond was last seen sitting inside a glass showcase at 5pm Thursday. Police say that an hour later, just after a closing time, the diamond was gone and the jewelry case was unlocked.

Police said on Saturday they suspect the alleged theft took place sometime in the final hour of the crowded exhibit at Yokohama, near Tokyo. Nobody has been arrested.

Investigators are checking security camera footage that showed a man reaching toward a showcase during the suspected time of theft.

Organisers said the three-day exhibit ended on Friday. About 410 jewellery shops from around the world and more than 10,000 visitors with tickets gathered.

Associated Press