Crown Jewel diamonds may be gems from deep within Earth, scientists say

Scientists believe that a diamond that figures prominently in British crown jewels may have originated from far below Earth’s surface, several times deeper than the region where most diamonds are formed.

Researchers at the Gemological Institute of America (GIA) in New York City say that the Cullinan diamond incorporated into the famous royal royalty collection may be a “super deep” gemstone.

Cut in South Africa, the Cullinan is the largest diamond ever found. It was delivered to King Edward VII in 1907 by the Transvaal government, a symbolic gesture intended to bridge the gap between Britain and South Africa after the Boer War.

The royal family refers to some of the other cut pieces of the original diamond: the Cullinan III, a 94.4-carat pear-shaped bead, and Cullinan IV, a 63.6-carat cushion-shaped diamond mounted as a clasp, such as ” Granny’s fries. “

Parts of it were later placed in the Crown Jewels, the Sovereign’s Scepter, and the Crown of the Imperial State.

Diamonds are formed under high pressure conditions in the Earth’s mantle, the intermediate layer between the planet’s crust and the nucleus.

Cullinan's brooch at the Queen's Gallery at Buckingham Palace
Cullinan’s brooch at the Queen’s Gallery at Buckingham Palace (Sean Dempsey / PA)

Most diamonds form at depths of 150 km to 200 km below Earth’s surface, at the base of continental tectonic plates.

But some rarer diamonds come from the depths of the mantle, below the rigid and stable continental plates, down, where the mantle moves slowly, what scientists call super deep diamonds.

Another diamond believed to be super deep is the famous Smithsonian Hope Diamond.

Scientists examined a 124-carat large diamond from the Letseng mine in Lesotho along with a blue diamond from a mine in South Africa.

The Lesotho stone is believed to come from the same class of diamond as the Cullinan, while the South African gem has similar characteristics to the Hope diamond.

The researchers speculate that the depth of origin of these diamonds could span 360 km to 750 km below Earth’s surface.

The GIA Dr. Evan Smith, who conducted the research, said: “We examined the first confirmed large gem diamonds that originate from Earth’s lower mantle, which is several times deeper than most other diamonds.

“The results support earlier predictions based on smaller gems, suggesting that diamonds with properties similar to those studied, including Cullinan and Hope diamonds, are super deep diamonds.”

The royal scepter, which is part of the jewels of the British crown, with a Cullinan diamond
The royal scepter, which is part of the jewels of the British crown, with a Cullinan (PA) diamond

The team also detected chemical debris from the mineral bridgmanite, which does not exist on Earth’s surface, in the diamonds they examined.

Dr. Smith said: “Finding these remains of the elusive Bridgmanite mineral is significant.

“It is very common on deep Earth, in extreme pressure conditions of the lower mantle, below a depth of 660 km, even deeper than most super deep diamonds.

“Bridgmanite does not exist in the upper mantle or on the surface.

“What we really see in diamonds when they reach the surface is not bridgmanite, but the minerals that remain when it decomposes as pressure decreases.

“Finding these minerals trapped in a diamond means that the diamond itself must have crystallized to a depth where bridgmanite exists, very deep within Earth.”

The findings were presented at the Goldschmidt 2020 geochemistry virtual conference and have not yet been peer-reviewed.