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What Makes Blue Diamonds So Precious?

A look at one of the most sought-after gemstones in the world, and some of the most exceptional blue diamonds in history, with a chance to get your own at the upcoming Sotheby's auction.

By Charlie Co
April 22, 2022

Which is the most precious gemstone of them all? A commonly asked question that usually ends with a heated debate that raises everything from quality, caratage, cut and colour, to rarity and origin. All valid benchmarks, no doubt, but the final verdict never quite so definitive. Weighing the importance of all these factors makes it a complicated assessment, not to mention the personal preferences that come into play.

But let’s set all those arguments and personal predilections aside for a moment and look at a stone that has captured the imagination and curiosity of connoisseurs and gemmologists alike: blue diamonds. It’s desired for its rarity, unique colour, and the mystery surrounding its colour—one that was only fully explained some five years ago.

Blue diamonds are formed at extreme depths reaching 660 km or more into the Earth’s mantle—that’s four times deeper than most diamonds. For the longest time, questions have been raised as to how it gets its colour, but in a research paper featured by scientific journal Nature in 2018, it was discovered after a 2-year study that the blue occurs because of the occurrence of boron, which is not naturally found in its pure elemental form on Earth but finds its way to the floors of ancient oceans carried deep into the Earth’s mantle by the movement of tectonic plates.

The De Beers Cullinan Blue being sold at Sotheby's upcoming auction on April 27 | Photo: De Beers

Blue diamonds are considered “Type IIb” diamonds, which are classified as being extremely uncommon and very highly valued. And while most blue diamonds are rare, those that have an exceptionally deep blue colour are the rarest. And if by any chance you come across large ones, which are even harder to come by, the value skyrockets.

The famous Hope Diamond meets both criteria. One of the world’s most famous specimens of this kind, it now sits behind the closely guarded glass pedestals of the distinguished Smithsonian Institute in Washington, DC. It weighs an astounding 45.52 carats and features a deep blue hue. Its exceptional physical attributes notwithstanding, it also owes its legendary status to its intriguing and mysterious history. The Hope Diamond is also one of the earliest mentions of blue diamonds in history, which is believed to have been first extracted in the Kollur mine, one of the famous Golconda diamond mines in India in the 17th century.

Today, blue diamonds are only found in a few mines around the world, including the Cullinan mine in South Africa, and the Argyle mine in Australia. The Golconda mines, which were known for having diamonds of exceptional clarity and size—blue diamonds included—are now depleted and have no new diamonds.

While no other blue diamonds have quite come close to the notoriety and prominence of the Hope Diamond, these five epic stones can be contenders, all of which are notable for their exceptional physical attributes or provenance.



Photo: The Blue Moon

The 12.03-carat ‘Blue Moon’ diamond went under the hammer for a staggering US$48 million at an auction in 2015. At about US$4 million per carat, it’s one of the most expensive diamonds per carat ever sold. Discovered in 2014 at the Premier mine in South Africa, it was given the highest possible colour grading for blue diamonds—The Gemological Institute of America (GIA) labelled it as ‘internally flawless,’ a perfect clarity grade—and it is often referred to as a smaller version of the Hope Diamond with its quality and similar cushion cut.



The Heart of Eternity | Photo: De Beers

Also discovered in the Premier Diamond mine and cut by the Steinmetz Group for De Beers, the Heart of Eternity is a fancy vivid blue diamond with a size of 27.64 carats. Making its debut in 2000 as part of the De Beers Millennium Jewels collection at the Millennium Dome in London, it was later displayed at the Smithsonian and is assessed to have a US$3 million per-carat value, or US$80 million.



Photo: Smithsonian Institution

At 30.62 carats, the Blue Diamond is the largest heart-shaped diamond on record and is often coined as “one of the prettiest diamonds in the world.” The Blue Heart is also sometimes called the "Unzue" diamond after the Argentinean woman, Mrs. Unzue, who purchased it from Cartier in 1910 and went on to own it for 43 years. The diamond is also commonly, but erroneously, referred to as the "Eugenie Blue," after Empress Eugenie of France, who could never have owned the diamond because of its much later discovery. After ownership passed several times, Marjorie Merriweather Post purchased the diamond from Harry Winston in 1960 and donated it to the Smithsonian four years after.



Photo: Graff

If there was a blue diamond that probably comes closest to having a storied past like the Hope Diamond, it would be the Wittelsbach-Graff, which was also found in the Kollur mine in India. The stone has a deep blue hue and is over 31 carats and was labelled internally flawless by the GIA. Once known as the Wittelsbach, the diamond—which used to be much bigger at 35.56 carats—was mounted on the crown of the King of Bavaria until 1918. In 2008, it was bought by jeweller Laurence Graff for US$23.4 million. Graff then re-cut it to the size it is today to improve its brilliance and colour, and its value is now estimated of US$80 million.



Photo: De Beers

In its rough original form, the De Beers Millennium Star Diamond weighed a staggering 777 carats when it was first found in the Congo (DRC), making it the 6th largest diamond in the world and insured for an eye-watering US$145 million. After more than 3 years of cutting and polishing to optimise the beauty of the stone, the Millennium Star Diamond is now a 203-carat, pear-shaped diamond and to date is the world’s largest top-colour internally and externally flawless pear-shaped diamond in the world.

If you’re dreaming of owning one of such gems, or at least curious to see one today, Sotheby’s is set to auction off another extraordinary De Beers blue diamond called “The De Beers Cullinan Blue,” a fancy vivid, 15.10-carat step-cut blue diamond graded as internally flawless by the GIA. It is estimated to go for at least US$48 million, and will be featured in a stand-alone, single lot live auction on April 27, 2022 right here in Hong Kong.