A 5.16-carat fancy, internally flawless, vivid blue diamond that was once part of the De Beers' famed "Millennium Jewels Collection" will lead the spring sale for Sotheby's Hong Kong, National Jeweler reported.

According to the report, the pear-shaped blue stone is expected to fetch $4.6-$5.8 million at the April 7 auction.

Although the Millennium blue diamond is currently part of a private collection, it was formerly a part of the Millennium Jewels Collection, an assortment of 12 extremely rare diamond assembled by De Beers and presented in 2000 to celebrate the new millennium.

The Steinmetz Diamond Group cut and polished all the diamonds in the collection and each of the blue stones was laser-inscribed with a De Beers millennium number.

The diamond is one of nine blue diamonds in the De Beers’ collection graded as "fancy vivid" – the highest color grade for a fancy color diamond. It is the first blue diamond from the collection to appear at auction.

According to a Sotheby's release, blue diamonds are among the rarest of all gemstones.

South Africa’s Premier Diamond Mine in Transvaal, is the world's main source of blue diamonds.