The gang did not take a 32-carat diamond worth more than pounds 2m and a pounds 500,000 necklace during the 15- minute raid on the Graff workshop, where jewellery is made for some of the world's richest people.
The three men, masked and carrying pistols, burst into the Graff workshop in Hatton Garden, central London, the centre of Britain's jewellery trade, at 8.45am and got through three combination-locked doors to reach the sorting room.
There they threatened and handcuffed Raymond Graff, brother of the company's owner, Laurence, his son and two employees, one of them a woman. Raymond, a director of the firm, runs the workshop.
They emptied the safe, taking a ruby and diamond necklace completed the previous day for sale at Graff's shop in Knightsbridge, west London, and dozens of loose diamonds.
But the most valuable part of their haul was a unique heart-shaped pure white 25-carat diamond the size of a pounds 1 coin, which is worth pounds 1.5m. Last night Laurence Graff pleaded with the robbers not to recut the stone, which was to have been the centrepiece of a necklace.
Within 15 minutes the gang had left, vanishing into the busy street. The handcuffed staff were shocked but unhurt and were freed a few minutes later by colleagues, some of whom had not realised the raid was going on.
A spokesman for Graff said: 'They seemed to know exactly what they were after. They targeted the sorting room. We have 35 people working on five floors but they only went to one. There was no break-in. The robbers got through three doors with combination locks.'
Although the company said yesterday that it believed that the stolen gems could be recovered, most people in the diamond trade were less confident. The most likely destination for the haul is Antwerp.
The workshop makes jewellery for the shop in Brompton Road, Knightsbridge. Laurence Graff founded the company 30 years ago and his clients reputedly include the Sultan of Brunei and several Arab royal families.
Police described the raiders as white, aged 25-35 and about six feet tall. Detectives questioned staff about the robbery and appealed for witnesses to come forward. Anyone with information is asked to telephone 071- 230 2061.Reuse content