A 5.16-carat Pear-Shaped Internally Flawless Fancy Vivid Blue Diamond Ring will be the prize piece of Sotheby’s upcoming “Jewels and Jadeite” Spring Sale, taking place on April 7 in Hong Kong.

This rare gem, which is from an Important Private Collection, is the first of De Beers Millennium Blue Diamond Collection to be at auction. Estimated at $4.6-5.8 million, this diamond was expertly cut and polished by The Steinmetz Diamond Group, and is laser-inscribed with the number “11″ on it; there were 12 diamonds in the Millennium Collection.

Of those 12, nine were given the grade of “Fancy Vivid” by the Gemological Institute of America, with this particular one being among them.

Blue diamonds are some of the rarest in the world, with the Premier Diamond Mine of Transvaal in South Africa being the main source. Out of all the rough diamonds that are excavated each year, it is said that only one of them will be able to produce a significant blue diamond.

The De Beers Millennium Collection was presented back in 2000 to celebrate the turn of the millennium. They were displayed in London’s Millennium Dome, as well as Japan and Dubai.

[Sotheby's]