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Scientists Solve Mystery Of How Rare Blue Diamonds Are Formed

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A team of scientists analyzes 46 blue diamonds to trace how deep in the earth do they come from and how exactly do they get their precious color.

Blue diamonds or type IIb diamonds are particularly rare. The scientists said barely one-hundredth of 1 percent of all diamonds in the world makes it to the cut of being classified as type IIb diamonds. One of the famous blue diamond is the Hope Diamond that sits at the Smithsonian National Museum of Natural History since 1958. The Hope Diamond has ownership records dating back almost four centuries. It had been in the possession of King Louis XIV of France.

Blue diamonds are tremendously expensive. The Cullinan Dream, which was sold at auction in 2016, was priced at more than $23 million. Fortunately, for the scientists, they were able to include this diamond in their analysis.

The Mystery Of The Blue Diamonds

The study, published in Nature on Aug. 1, found that type 11b diamonds were formed deep into the Earth, at least as deep as the transition zone between the planet's upper and lower mantle. Interestingly, they got their precious blue color because of boron, an element that is found on the Earth's surface.

To be exact, blue diamonds were formed deep between 410 and 660 kilometers below the surface. Some of the diamonds analyzed for the study showed indications that they came from a location deeper than 660 kilometers. The most common gems can be fetched from between 150 and 200 kilometers.

If blue diamonds are found this deep into the Earth's mantle, how did they come in contact with boron, which was abundant on the Earth's surface?

Solving The Puzzle

The team of scientists, headed by Evan Smith of the Gemological Institute of America, concluded that the boron seeped from the seafloor that was pushed down into the Earth's mantle through a process called subduction, or when one tectonic plate slid beneath another. The boron merged into water-rich minerals that crystallized after undergoing geochemical reactions that take place between seawater and the rocks found in the oceanic plate.

Aside from finally being able to explain how rare blue diamonds got their color, another takeaway from the study is the discovery that water-bearing minerals can travel far deeper into the Earth's mantle. This is new information among the scientific community.

"This new discovery that blue diamonds also have super-deep origins, we now know that the finest gem-quality diamonds come from the farthest down in our planet," said Steven Shirey, one of the authors for the study.

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