The operation was linked to the robbery at Selfridges in London's Oxford Street and another at a jeweller's in Windsor, Berkshire, in April:
This morning's raids represents the next stage in what is a complex and ongoing investigation.
The actions of the people involved in these two offences showed a blatant disregard for the safety of the public and those members of staff at the premises targeted.
Today's arrests show that the Flying Squad will continue to pursue those who choose to commit these types of offences, gathering the necessary intelligence to arrest them and bring them before the courts.
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Although the diamond was believed to be cut in the 19th Century, its history and how it arrived at the car boot sale is unknown.