Blue diamond in the rough is off the scale

Once in a blue moon is a way of saying something happens only very rarely. So when a South African mine announced it had found a large rough blue diamond, there was really no other name for the polished stone.

“Only 0.01 per cent of the entire production of the [Cullinan mine] is any shade of blue and this stone is vivid blue so it really is off the scale,” says David Bennett, worldwide chairman of Sotheby’s international jewellery division, adding that only a handful of blue diamonds of this size and quality exist.

The 12.03-carat flawless Blue Moon diamond, which started out as a 29.6-carat rough stone, is expected to fetch $35 million to $55m when it is auctioned in Geneva on November 11.


The stone comes from the same mine where the world’s largest rough diamond, the 3,106 carat Cullinan Diamond, was found in 1905.

The estimate for the Blue Moon diamond – named after an astrological event in which there are two full moons in a calendar month – is about $3m a carat; a colourless, flawless stone of the same size which would sell at about $130,000 per carat.

The stone is internally flawless and was described as an almost indefinable colour by the Gemological Institute of America, the body which grades stones.

“It is absolutely not the blue of a sapphire. It is more of an ocean blue. It is an absolutely mysterious and magical stone,” says Mr Bennett.

The rough stone was bought by Cora International, a New York-based firm which specialises in cutting large diamonds, for $25.6m. It was cut and polished for more than six months.

“I have sold so many of these famous diamonds,” says Mr Bennett, “but what I particularly like is these hugely important new stones, because I feel like a bit like a midwife. Because from the point of being in the earth for millions of years to this very brief moment of being cut and polished, it then passes through Sotheby’s and into history when it becomes owned by somebody and all these people over the next hundreds of thousands of years will think it is theirs.”

Q&A

David Bennett, worldwide chairman of Sotheby’s international jewellery division, reveals more about the Blue Moon diamond:

Is it expected to be the most expensive blue diamond ever sold?

We will wait and see. We sold a 9-carat vivid blue diamond in December last year for about $3.3m per carat. That’s where they are the moment.

What makes blue diamonds blue?

The blue is actually a result of an impurity. Billions of years ago the boron was incorporated into the crystal lattice, imparting this blue colour. So it is really a freak of nature. But of course this was happening several miles below the Earth’s surface when it was being formed, way, way, way beyond where man could possibly mine. The reason that we have it, as with all diamonds, is that it got associated with a volcano so it was carried up by the shaft of a volcano.

Is 12 carats too big to wear in a ring?

I have had 100-carat diamonds made into a ring. Twelve carats is actually the perfect size. In fact we have mounted it as a ring for the exhibition.

Will it be exhibited in the Middle East?

It won’t be coming to the Middle East but it will be on exhibition in London next week and many of our Middle East clients visit London on a regular basis, so hopefully they will catch it there.

Have you had any interest from anyone here?

There have been a lot of requests for catalogues, but generally buyers keep their cards very close to their chests. They don’t want to have competition.

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