Prosecutors said they are searching for a man who broke into open safety deposit boxes in an ABM Amro bank in the city's diamond quarter.
Police are unsure how he did it but believe he may have carefully planned the robbery, becoming one of several trusted diamond traders given an electronic card to access the vault. The suspect had been a regular customer at the bank for the past year, giving the name Carlos Hector Flomenbaum from Argentina.
They now believe that the suspect was using a false identity because a passport in that name was stolen in Israel a few years ago.
They released a composite photo of a gray-haired man, 6 foot, 3 inches tall and aged between 55 and 60. They said he speaks English with an American accent and often wears a baseball cap, and they are appealing to anyone who got to know him during his time in the city to come forward.
The bank discovered the theft on March 5, believing that someone took the stones that Monday morning or the previous Friday from a vault used by pawnbrokers and diamond cutters.
Police did not say why they had waited over a week before making the theft public, nor did they mention who had put up the reward money.
The stolen diamonds weigh 120,000 carats and include some very unusual stones, police said.
The diamonds will be difficult to sell on world markets, said Philip Claes, spokesman for the Antwerp Diamond Council. He told Belgium's RTL-TVI television network the haul included both cut gems and rough diamonds which cannot be sold without certificates of origin to show they have not come illegally from war zones.
In 2003, in the world's largest safe-deposit box theft, thieves in Antwerp pried open 123 boxes, finding so much loot they could only carry away $100 million worth of diamonds, gold and jewelry.
More than half the world's diamonds are traded in Antwerp's gem district, a maze of tiny streets hugging the main train station. Its turnover of $23 billion a year makes it one of the densest concentrations of valuables on earth.