30 Jun 2016

Largest uncut diamond set for sale

11:10 am on 30 June 2016

The second-biggest rough diamond ever found is on sale, one of three big diamonds found recently after a break of more than a century.

The Lesedi La Rona, a tennis ball-sized gem found in Botswana, had been estimated to sell for over $70 million.

The Lesedi La Rona, a tennis ball-sized gem found in Botswana, had been estimated to sell for over $70 million. Photo: AFP

On 16 November 2015 Tiroyaone Mathaba was sorting through rocks at the Karowe mine in Botswana when he spotted what turned out to be the 1109 carat diamond believed to be more than 2.5 billion years old.

It was the size of a tennis ball.

His boss, William Lamb, said all of the mine's 804 employees and contractors received a bonus, but he did not reveal how much.

Mr Mathaba's discovery, The Lesedi la Rona which means "our light" in the Tswana language, was not the only big gem discovered that week - two other huge diamonds were found from the same mine within 72 hours.

A 374 carat diamond, which has not been named, and an 812.77 carat diamond called The Constellation were also discovered.

Mr Mathaba was working for Canadian company, Lucara which suspected there might be big diamonds in the area after recovering a 239 carat diamond two years earlier.

At that time it was standard practice to break down any kimberlite ore to 30mm before putting it through the sorting machine to test its density and see if it contained any diamonds.

1984 photo of the Zale diamond, largest diamond-in-the-rough in the world, weighing 890 carats with an irregular shape and yellow colour.

1984 photo of the Zale diamond, former largest diamond-in-the-rough in the world, weighing 890 carats with an irregular shape and yellow colour. Photo: AFP

The machine could not cope with anything bigger.

But this raised the possibility that large diamonds were being crushed in the very process of trying to discover them.

So Lucara bought an X-ray machine to detect the carbon in the ore, and the investment paid off.

The Constellation sold for $US63.1m in May, and The Lesedi la Rona was expected to be sold for even more - £52m - at the Mayfair auction at Sotheby's in London on Wednesday.

The two were the biggest diamonds ever recovered through an industrial process, Lucara's chief operating officer Paul Day said.

The biggest diamond, the Cullinan, was found more than 100 years ago only nine meters from the surface and, the story goes, was extracted using a pocket knife, Mr Day said.

It originally weighed 3106.75 carats, and was cut into several gems including the Cullinan I which was mounted into the British royal jewels.

That's a far cry from the modern industrial mining techniques which used explosions to extract the ore, and crushing machines to break it down further.

Mr Day said he found the gap of more than a century between finding the largest and second-largest diamond surprising.

He pointed out several large diamonds were also found before industrial mining began.

He said it was not out of the question that bigger diamonds could have previously been crushed down to smaller pieces.

"Diamonds are incredibly hard and also very strong but they do break."

"If you take a 100 carat diamond and you hit it with a hammer against steel there is a chance you will break it into pieces."

This kind of force is what the rocks undergo in the mining process.

Mr Day called it the diamond miner's dilemma - there's not much that can be done to reduce the damage done to the diamonds while extracting them from the ground, he said.

But changing the sorting process would leave diamond miners one step closer to having just one dilemma: How to spend the money.

According to a study by the Gemological Institute of America, the Lesedi La Rona diamond's colour and transparency "exemplify" type IIA diamonds - "the most chemically pure and often show extraordinary optical transparency".

David Bennett, worldwide chairman of Sotheby's jewellery division, said the diamond's discovery is "the find of a lifetime".

"Every aspect of this auction is unprecedented. Not only is the rough superlative in size and quality, but no rough even remotely of this scale has ever been offered before at public auction," he said.

Lesedi La Rona measures measures 6.64 x 5.5 x 4.2cm and in terms of its size is exceeded only by the Cullinan Diamond, mined in South Africa in 1905 and presented to King Edward VII.

The 3106-carat diamond was cut into nine separate stones, many of which are in the British Crown Jewels, including the Great Star of Africa - currently the largest top-quality polished diamond in existence.