He then "calmly" left the Harry Winston store.
A robber took $16 million in diamonds from a jewelry store in Cannes, France, authorities said.
The man, armed with a handgun and a grenade, then "calmly escaped on foot," Reuters reported. It's not yet clear whether the grenade was genuine.
The Associated Press reports that the store was a Harry Winston location. The merchandise stolen included diamond necklaces along with other jewelry, according to AP.
The robber initially posed as a customer before showing the gun an instructing two salespersons to open the jewelry cases, Reuters reported. The culprit wore sunglasses but otherwise did not hide his face, a prosecutor said.
No injuries were reported in the robbery.
Cannes is located on the French Riviera. The store is located within the city's Croisette waterfront area.