By Annemarie Evans
BBC News, Hong Kong
The blue diamond is rare, flawless and 'fancy vivid'
A rare and flawless blue diamond is to be offered for auction in Hong Kong.
The diamond, originally from the De Beers Millennium Collection, is expected to sell for between $4.6m and $5.8m (£3m and £3.8m) at Sotheby's.
It is one of 12 in a collection the diamond company says took decades to put together.
Brought together to celebrate the new millennium, the collection was displayed in London's Millennium Dome in 2000.
An attempt to steal the diamonds was thwarted by Metropolitan police.
The blue pear-shaped diamond is the first piece from that collection to be auctioned, and according to Sotheby's is currently owned by an important private buyer.
The collection has 11 blue diamonds, polished by the Steinmetz Diamond Group.
Out of the 11, nine - including the one up for auction later on Wednesday - are what are called fancy vivid, the highest grade level for a colour diamond.
It is no surprise that Sotheby's has chosen Hong Kong to sell the De Beers Millennium Blue Diamond.
As the Chinese economy grows, buyers have shown an increasing appetite for high-end luxury goods.