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Giant diamond whose cuts adorn the Crown Jewels was formed 400 miles below the Earth surface

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The giant diamond cut into gems that now adorn the Crown Jewels was formed 400 miles below Earth’s surface — three times deeper than other precious stones. 

Analysis of similar diamonds by the Gemological Institute of America revealed that the Cullinan was a ‘super-deep’ diamond — and one of the rarest objects on Earth.

The largest gem-quality rough diamond ever found, weighing 3,106.75 carats, the Cullinan was unearthed from a mine in South Africa in the January of 1905.

In 1907, the diamond was bought by the Transvaal Colony government, who presented it to King Edward VII as a gift.

The King had the rough stone cut by Joseph Asscher & Company of Amsterdam — forming nine major stones (Cullinan I–IX) as well as 96 minor brilliant stones.

The largest two stones — Cullinan I and II — now form centrepieces in the Crown Jewels, and sit in the Sovereign’s Sceptre with Cross and the Imperial State Crown.

Despite originally remaining in Amsterdam, the other seven major stones have over time also been acquired or gifted into the possession of the British royal family.

The recent study also concluded that the famous Hope Diamond — presently held in the collections of the Smithsonian Museum in the US — may also be ‘super deep’.

The giant diamond cut into gems that now adorn the Crown Jewels was formed 400 miles below Earth’s surface — three times deeper than other precious stones. Pictured, the Crown Jewels of the United Kingdom, which contain the largest stones cut from the Cullinan diamond

‘We examined the first large gem diamonds confirmed to originate from Earth’s lower mantle, which is several times deeper than most other diamonds,’ said Gemological Institute of America geologist Evan Smith.

‘The results support earlier predictions, based on smaller gems, suggesting that diamonds with properties similar to those studied — including both the Cullinan and Hope Diamonds — are super-deep diamonds.’

Diamonds are formed under high pressures within the Earth’s mantle — the middle layer that lies between the surface crust and the central core.

While the majority of diamonds form in the base of continental tectonic plates — at depths of up to around 125 miles — some rarer diamonds form deeper in the mantle.

These ‘super-deep’ diamonds originate beneath the rigid and stable continental plates, down at depths where the mantle is slowly moving, or ‘convecting’.

The Hope Diamond is classified as a ‘type IIb’ diamond — one which contains the element boron and gives the Hope its noted blue tinge.

Until now there has been uncertainty as to whether ‘type IIb’ diamonds formed in a shallow or deep environment — particularly as concerned those that are larger than three carats, which is abound the size of a pea.

It is only within the past couple of years that scientists have begun to understand exactly where in the Earth these dazzling blue gemstones form.

Now, working from their lab in New York, Dr Smith and his colleague Wuyi Wang have detected the remains of a characteristic mineral called ‘bridgmanite’ in a large, 20 carat, type IIb diamond from a mine in South Africa.

Diamonds are formed under high pressures within the Earth's mantle — the middle layer that lies between the surface crust and the central core, pictured. While the majority of diamonds form in the base of continental tectonic plates — at depths of up to around 125 miles — some rarer diamonds form deeper in the mantle

Diamonds are formed under high pressures within the Earth’s mantle — the middle layer that lies between the surface crust and the central core, pictured. While the majority of diamonds form in the base of continental tectonic plates — at depths of up to around 125 miles — some rarer diamonds form deeper in the mantle

The recent study also concluded that the famous Hope Diamond, pictured — presently held in the collections of the Smithsonian Museum in the US — may also be'super deep'

The recent study also concluded that the famous Hope Diamond, pictured — presently held in the collections of the Smithsonian Museum in the US — may also be ‘super deep’

The recent study also concluded that the famous Hope Diamond — presently held in the collections of the Smithsonian Museum in the US — may also be'super deep'. Pictured, original illustrations of the Tavernier Blue diamond, which would be cut into the Blue Diamond of the French Crown and eventually the Hope Diamond

The recent study also concluded that the famous Hope Diamond — presently held in the collections of the Smithsonian Museum in the US — may also be'super deep'. Pictured, a cubic zirconia replica of the Tavernier Blue diamond, which would be cut into the Blue Diamond of the French Crown and eventually the Hope Diamond

The recent study also concluded that the famous Hope Diamond — presently held in the collections of the Smithsonian Museum in the US — may also be ‘super deep’. Pictured, original illustrations (left) and a cubic zirconia replica (right) of the Tavernier Blue diamond, which would be cut into the Blue Diamond of the French Crown and eventually the Hope Diamond

By aiming a laser at tiny inclusions of other minerals trapped inside the diamond and measuring the characteristic way the light was scattered, the duo determined that these minerals had been left behind by the breakdown of bridgmanite.

‘Finding these remnants of the elusive mineral bridgmanite is significant’, explained Dr Smith.

‘It’s very common in the deep Earth at the extreme pressure conditions of the lower mantle, below a depth of 660 kilometres [410 miles], even deeper than most super deep diamonds. 

‘Bridgmanite doesn’t exist in the upper mantle, or at the surface.’

‘What we actually see in the diamonds when they reach surface is not bridgmanite, but the minerals left when it breaks down as the pressure decreases.’

‘Finding these minerals trapped in a diamond means that the diamond itself must have crystallised at a depth where bridgmanite exists, very deep within the Earth’.

The largest gem-quality rough diamond ever found, weighing 3,106.75 carats, the Cullinan (pictured) was unearthed from a mine in South Africa in the January of 1905. In 1907, the diamond was bought by the Transvaal Colony government, who presented it to King Edward VII as a gift. The King had the rough stone cut by Joseph Asscher of Amsterdam — forming nine major stones (Cullinan I–IX) as well as 96 minor brilliant stones

The largest gem-quality rough diamond ever found, weighing 3,106.75 carats, the Cullinan was unearthed from a mine in South Africa in the January of 1905. In 1907, the diamond was bought by the Transvaal Colony government, who presented it to King Edward VII as a gift. The King had the rough stone cut by Joseph Asscher (pictured) of Amsterdam — forming nine major stones (Cullinan I–IX) as well as 96 minor brilliant stones

The largest gem-quality rough diamond ever found, weighing 3,106.75 carats, the Cullinan (pictured left) was unearthed from a mine in South Africa in the January of 1905. In 1907, the diamond was bought by the Transvaal Colony government, who presented it to King Edward VII as a gift. The King had the rough stone cut by Joseph Asscher (right) of Amsterdam — forming nine major stones (Cullinan I–IX) as well as 96 minor brilliant stones

‘We also examined a large 124 carat diamond from the Letseng mine in Lesotho,’ Dr Smith said, referring to the enclaved kingdom within the borders of South Africa.

‘This diamond — which is around the size of a walnut — is very pure, containing no nitrogen in its crystal structure, and is known as a “CLIPPIR” [or Cullinan-like, Large, Inclusion-Poor, Pure, Irregular, and Resorbed] diamond.’

‘This is from the same class of diamond as the famous Cullinan Diamond, which is now the centrepiece of the British Crown Jewels.’

‘This large diamond showed the same characteristic bridgmanite breakdown products, meaning that it too had been formed as a super-deep diamond.’

‘What is special about this one is that it is the first CLIPPIR diamond for which we can firmly assign a lower mantle origin — that is, below 660 kilometres [410 miles].’

‘Previously, we had known that CLIPPIR diamonds are super-deep and speculated that their depth of origin might span 360–750 kilometres depth, but we hadn’t actually seen any that were definitely from the deeper end of this window.’

‘This gives us a better idea of exactly where CLIPPIR diamonds, such as the Crown Jewel diamonds, come from.’

‘What we have learned here is that there is some overlap in the birthplace for CLIPPIR diamonds, such as the Cullinan, and type IIb diamonds, such as the Hope. This is the first time this has been found.’

King Edward VII had the Cullinan stone cut by Joseph Asscher of Amsterdam — forming nine major stones (Cullinan I–IX, pictured) as well as 96 minor brilliant stones

King Edward VII had the Cullinan stone cut by Joseph Asscher of Amsterdam — forming nine major stones (Cullinan I–IX, pictured) as well as 96 minor brilliant stones 

The largest two stones — Cullinan I and II (top centre and top left) — now form centrepieces in the Crown Jewels, and sit in the Sovereign's Sceptre with Cross and the Imperial State Crown. Despite originally remaining in Amsterdam, the other seven major stones (also pictured) have over time also been acquired or gifted into the possession of the British royal family

The largest two stones — Cullinan I and II (top centre and top left) — now form centrepieces in the Crown Jewels, and sit in the Sovereign’s Sceptre with Cross and the Imperial State Crown. Despite originally remaining in Amsterdam, the other seven major stones (also pictured) have over time also been acquired or gifted into the possession of the British royal family

Boron-rich type IIb diamonds — such as the Hope Diamond — are very rare. In fact, less than one-in-a-thousand diamonds are classified as being type IIb.

‘Discovering the deep mantle origin means that the material in these diamonds undergoes a remarkable journey,’ Dr Smith said.

‘We believe that the boron — which give the Hope Diamond it’s characteristic blue colour — originates from the bottom of the oceans.’

‘From there, plate tectonics drags it hundreds of kilometres down into the mantle, where it can be incorporated into diamond.’

‘It shows that there is a gigantic recycling route that brings elements from Earth’s surface down into the Earth, and then occasionally returns beautiful diamonds to the surface, as passengers in volcanic eruptions’.

The Cullinan Diamond produced stones of various cuts and sizes, the largest of which is named Cullinan I — or the'Great Star of Africa' — and, at 530.4 carats (106.08 g), is the largest clear-cut diamond in the world. The stone is typically mounted in the head of the Sovereign's Sceptre with Cross but can also be removed and worn as part of a brooch along with Cullinan II, a 317.4 carat (63.48 g) stone set in the Imperial State Crown. This setting was often favoured by The Queen's grandmother, Mary of Teck, pictured here wearing this configuration

The Cullinan Diamond produced stones of various cuts and sizes, the largest of which is named Cullinan I — or the ‘Great Star of Africa’ — and, at 530.4 carats (106.08 g), is the largest clear-cut diamond in the world. The stone is typically mounted in the head of the Sovereign’s Sceptre with Cross but can also be removed and worn as part of a brooch along with Cullinan II, a 317.4 carat (63.48 g) stone set in the Imperial State Crown. This setting was often favoured by The Queen’s grandmother, Mary of Teck, pictured here wearing this configuration

The seven other major diamonds cut from the Cullinan — which weigh a total of 208.29 carats (41.66 g) — are also privately owned by The Queen, who inherited them from her grandmother in 1953. Pictured, The Queen, seen here with the Imperial State Crown and wearing a brooch containing Cullinan III. In 1958, her majesty revealed during a visit to the Netherlands that the Cullinan III and IV are affectionately known among the present royal family as'Granny's Chips'

The seven other major diamonds cut from the Cullinan — which weigh a total of 208.29 carats (41.66 g) — are also privately owned by The Queen, who inherited them from her grandmother in 1953. Pictured, The Queen, seen here with the Imperial State Crown and wearing a brooch containing Cullinan III. In 1958, her majesty revealed during a visit to the Netherlands that the Cullinan III and IV are affectionately known among the present royal family as ‘Granny’s Chips’

‘This fascinating work confirms that the Hope Diamond is extraordinary and special, and truly one of Earth’s rarest objects,’ said geologist Jeff Post of the Smithsonian National Museum of Natural History, which holds the Hope within its collections.

The Cullinan Diamond produced stones of various cuts and sizes, the largest of which is named Cullinan I — or the ‘Great Star of Africa’ — and, at 530.4 carats (106.08 g), is the largest clear-cut diamond in the world.

The stone is typically mounted in the head of the Sovereign’s Sceptre with Cross — part of the Crown Jewels — but can also be removed and worn as part of a brooch along with Cullinan II, a 317.4 carat (63.48 g) stone set in the Imperial State Crown.

This setting was often favoured by The Queen’s grandmother, Mary of Teck.

The seven other major diamonds cut from the Cullinan — which weigh a total of 208.29 carats (41.66 g) — are also privately owned by The Queen, who inherited them from her grandmother in 1953.

The full findings of the study were presented at the 2020 Goldschmidt geochemistry conference, which is being held virtually from June 21–26.

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