Rare blue diamond set to fetch £3.8m

A rare internally flawless blue diamond is expected to fetch up to £3.8 million when it is sold at auction next month.

The 5.16-carat, pear-shaped stone, which is set in a ring, is the first blue diamond from the De Beers Millennium Jewels collection to go under the hammer.

It was among 12 stones displayed at the former Millennium Dome in 2000, in a collection which included the 203.04-carat Millennium Star white diamond, and 11 blue stones of various shapes and weights.

The gem will be sold by Sotheby's in Hong Kong on April 7, with an estimated price tag of 4.6 to 5.8 million US dollars (£3.04 to £3.8 million).

It has been graded as having "fancy vivid blue" colour, which is the highest for coloured diamonds.

Blue diamonds get their colour if the element boron is present when the stone is formed. They are extremely rare.

Quek Chin Yeow, from Sotheby's Asia, said: "It is an unprecedented opportunity to acquire at auction a gem with this unique historical provenance."

Each stone in the collection was laser-inscribed with a De Beers Millennium number, and this blue diamond is "De Beers Millennium Jewel 11".

Sotheby's sold a 7.03-carat internally flawless blue diamond, called the Star of Josephine, in May last year which fetched 9.48 million US dollars (£6.27 million).

At the time it set a world record auction price per carat of 1,349,751 million US dollars (£892,000).