The deepest in the world, the rarest diamonds turned out to be a big secret about our planet’s interior

The dark spots that are captured in rare blue diamonds as these contain valuable secrets about the Earth’s inner life. In a new study, scientists took an intimate look at 46 of the most expensive gems on Earth. Credit: Evan M. Smith/Copyright 2018 GIA

If you want to see how the world’s most famous diamonds looked like as babies, you will have to rewind history, with approximately a billion years.

Take a look at the beautiful, possibly cursed Hope Diamond — the spectacular, blue, 45-carat stone that has been permanently exhibited in the Smithsonian’s National Museum of Natural History since 1958 — and press rewind. As the years reel back, you can see the beautiful blue diamond leave the museum pedestal and quickly change the hands of merchants to the members of the beau monde for thieves to King Louis XIV. Eventually you will see a French gem merchant to send it back to a mine in India, where he is trapped in hardened lava maybe hundreds of thousands of years.

You will see that rock melting back into the smoldering magma, sucked into an exploding volcano and shoved deep, deep in the Earth’s mantle where the diamond will be slow to split into its constituent elements. Then, if millions of years pass like seconds, you will see a number of those elements rise to hundreds of kilometers back to the earth’s surface and the rest on the bottom of the sea. [Gallery: 13 Mysterious and Cursed Gemstones]

This is where the story begins in an exciting new paper, published today (Aug. 1) in the journal Nature. In a first-of-its-kind research, geological researchers from the United States, Italy and South Africa analyzed 46 of the world’s most valuable diamonds to answer a simple question: How deep in the earth, the precious blue diamonds form, and how do they do that? In investigating these questions, the researchers discovered that blue diamonds are not only some of the rarest and deepest of diamonds on Earth, but they can also contain the secrets of our planet’s interior that science is only beginning to scratch the surface of.

The rarest stones on Earth

Blue diamonds — or-type IIb diamonds are extremely rare. Barely a hundredth of 1 percent of all mined diamonds fit in with this classification, the authors of the study wrote. They are also very expensive.

“This so-called type IIb diamonds are the most valuable assets, which makes it difficult to access for scientific purposes,” lead study author Evan Smith of the Gemological Institute of America said in a statement.

In the current study, Smith and his colleagues spent two years to get acquainted with the most expensive blue diamond, including the so-called Cullinan Dream, an 24.18-carat diamond that sold at auction for more than $23 million in 2016.

The team screened hundreds of thousands of diamonds to pick their final line-up of glittering subjects, eventually choosing diamonds that showed clear inclusions visible spots of leftover minerals from the old, underground rocks where the diamonds are formed. By studying these inclusions carefully, researchers can estimate which types of minerals in the rocks where the diamonds are formed, the inclusions may also indicate where (approximately) in the earth’s crust, diamonds are formed.

With the help of Raman spectroscopy (a method of the scattering of lasers on a target to determine the unique molecular composition), the team determined that the inclusions in blue diamonds looked like rocks that can only be done in the Earth’s lower mantle, about 250 miles to 410 miles (410 to 660 kilometers) below the earth’s surface — about four times deeper than previously thought. By contrast, the researchers wrote, most of the other gem-quality diamonds come from about 90 km up to 125 miles (150 to 200 km below the ground. This makes the blue diamonds are not only some of the rarest, but also the deepest diamonds known on Earth.

“We now know that the best gem-quality diamonds come from the furthest down on our planet,” study co-author Steven Shirey, a researcher at the Carnegie Institution for Science in Washington, D. C., said in the statement.

For many of us, that is never the possession of a blue diamond, there is an intriguing silver lining. These findings also suggest that our planet recycles the surface minerals much deeper in the mantle of the earth than was previously thought.

Blue diamonds possess their colour from boron, an element that is found almost exclusively on the earth’s surface and in the underwater mineral deposits, the researchers wrote. To reach the dizzying depths where the blue diamonds are now thought to form, that boron probably driving the Earth’s dense oceanic crust underground when it collides with the continental crust at subduction zones — places where two tectonic plates smash together, giving the poet plate to sink below the less dense.

Because some of the inclusions in the blue diamonds were also surrounded by bags of hydrogen and methane, it is likely that some underwater mineral running drill in the mantle bore traces of the ocean water, too. This capability of the highlights of a potentially important route for ultra-deep recycling of water on Earth,” the researchers wrote.

To tighten further this hypothesis, researchers will need to study more of the world radiant, or diamonds. Never say the science is not glamorous.

Originally published on Live Science.



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