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Rare blue diamonds reveal secrets from hundreds of miles below the Earth's surface

Rare blue diamonds reveal secrets from hundreds of miles below the Earth's surface
These blue diamonds contain boron, and their mineral inclusions may help scientists understand how the Earth recycles its water and its crust. The gem on the left is 3.81 carats, and the one on the right is 2.08 carats. (Robison McMurtry and Jae Liao / Gemological Institute of America)

Blue diamonds are among the most rare and valuable gems on Earth, but up until now, scientists knew very little about them.

The most famous blue diamond is the Hope Diamond, which was discovered in the mid-1600s in India. It is now part of the gem and mineral collection at the National Museum of Natural History in Washington.

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Other blue-tinted diamonds have been found in southern and central Africa, South America and Borneo.

But there are very few of them around. Of all the diamonds that have made it to the surface of the Earth, less than 0.02% are blue diamonds.

Known in the scientific community as type IIb diamonds, blue diamonds get their telltale color from small amounts of boron that were locked into their crystalline structures when the diamond first formed.

“The diamond crystal is made up of all these carbon atoms linked together, but if you take one carbon out of every 1 million and replace it with boron, that’s still enough to make the diamond blue,” said Evan Smith, a geologist and research fellow at the Gemological Institute of America in New York.

It is well known that blue diamonds contain boron, but this fact has long been puzzling to geologists. Although boron is abundant in the Earth’s crust, it is scarce in the planet’s mantle where diamonds are formed.

“Having a diamond that is distinguished by its boron content immediately raises questions about where the boron comes from and how it got there,” Smith said.

In a paper published Wednesday in Nature, he describes this conundrum as a “geochemical enigma.”

Studying blue diamonds is challenging in part because they are so rare, and also because they are very expensive. It’s hard for researchers to get their hands on them.

However, as a scientist working at the Gemological Institute of America, Smith was able to examine 46 type IIb diamonds that were among the thousands of diamonds that come through the GIA’s offices each day to be graded according to what is known as the four Cs — cut, clarity, carat weight and color.

All rocks have a story to tell, but diamonds can be tight-lipped.

Diamonds themselves don’t reveal much information about the conditions in which they were born, but they often capture tiny bits of their surrounding environment during their formation.

These are known as inclusions, and although they are very tiny, they are much more forthcoming.

To determine the chemical makeup of a diamond inclusion, scientists usually shine a laser at it. The way the light gets scattered back tells them what molecules are in the inclusion, and the diamond remains unharmed.

Smith asked his colleagues to let him know when a blue diamond came across their desks and if it had inclusions. If it did, he’d ask to borrow it for a while. Then he’d hit its inclusions with a laser. Within a few hours, he returned the diamond to the stream of gems being graded.

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Gradually, over the course of two years, the inclusions in the blue diamonds that passed through Smith’s laser began to tell a cohesive narrative.

Their chemical makeup suggested that they formed at least 410 miles beneath the Earth’s surface, making them among the deepest-known diamonds in the world. (The vast majority of diamonds come from depths of 90 to 120 miles beneath the Earth's surface.)

In addition, he found that the mineral environment in which the diamonds were born is most similar to what you would expect to find in rocks from the ocean floor that have been subjected to an immense amount of pressure.

So, what’s going on here?

Smith thinks the most likely answer is that blue diamonds form in or around oceanic plates that have sunk deep into the Earth’s mantle.

This isn’t as crazy as it sounds. When two tectonic plates converge, oceanic plates are known to sink back into the mantle in a process called subduction.

“It’s been happening for millions of years and we’ve known it’s been happening,” Smith said.

What scientists didn’t know is what materials from the Earth’s surface these oceanic rocks were carrying down into the mantle.

The presence of boron in the blue diamonds suggests that at least that element was able to hitch a ride with the subducted oceanic plates into the deep mantle, Smith said. And if boron hitched a ride, it’s possible that water molecules that have reacted with the oceanic rock are being carried into the deep mantle in a similar way.

“The most likely explanation is that boron and water traveled hand in hand on this conveyor-belt-like system, but that’s still an assumption,” he said.

He added that more analysis of the chemical makeup of inclusions in blue diamonds may shed more light on the presence of water in the deep mantle.

“We think of these diamonds as gemstones, but they have this equally fascinating life as a scientific mineral,” he said. “There isn’t any other mineral that comes from so deep in the Earth that can bring up these small samples trapped inside and so well preserved.”

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