The blue diamond may not be the biggest gem ever sold, but it was very, very expensive

A London jeweller smashed a 20-year-old world record today by paying more than a million dollars per carat for a flawless blue diamond.

Alisa Mousaieff bought the gem for £3.91 million at an auction in Hong Kong after obsessively watching the rare blue gem change hands for more than two decades.

The diamond went for $1.32 million (£650,000) per carat beating the previous record set by the “Hancock Red” diamond, which fetched $926,000 (£456,000) per carat in New York in April 1987.

It will be brought back to London but not for display to the public. The stone will be locked away in the vaults at Moussaieff Jewellers in Mayfair. Ms Moussaieff, the owner, explained that it will be shown only to “legitimate hard-nosed business clients”.