Why have so many huge diamonds been found recently?

Image source, Donald Bowers
Image caption,
The second biggest ever rough diamond was discovered in Botswana

The huge rough diamond that went on sale at auction on Wednesday was one of three big diamonds found recently after a break of more than a century. The BBC's Clare Spencer asks why.

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 1,109 carat diamond.

It was the size of a tennis ball.

His boss William Lamb told True Africa that all of the mine's 804 employees and contractors received a bonus, but he didn't reveal how much.

Mr Mathaba's discovery, The Lesedi la Rona which means "our light" in the Tswana language, wasn't 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 hasn't been named and an 812.77 carat diamond called The Constellation were also discovered.

Mr Mathaba works for Canadian company Lucara which suspected that 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.

Image source, Lucara diamonds
Image caption,
This machine X-rays diamonds before they are crushed

The machine couldn't cope with anything bigger.

But this raises 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.

The investment paid off.

The Constellation was sold for $63.1m (£46m) in May.

The Lesedi la Rona had been expected to sell for even more on Wednesday but did not meet the minimum reserve price.

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

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

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

Image source, Getty Images
Image caption,
The Cullinan was sent from South Africa to the UK and broken down for the Queen's jewellery

Mr Day finds the gap of more than a century between finding the largest and second largest diamond surprising.

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

He says that it is not out-of-the question that bigger diamonds 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.

It is what Mr Day calls 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 says.

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

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