The Okavango Blue Diamond, and a Lesson in Geology

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Few of the visitors who marvel at the Okavango Blue Diamond on display at the American Museum of Natural History in New York will see a history lesson in geology and plate tectonics in its 20.46 carats. But scientists do.

The oval brilliant-cut fancy diamond, which has a clarity grade of VVS1, meaning it has very, very few slight inclusions, or imperfections, was discovered by the Okavango Diamond Company in 2018 at the sprawling Orapa mine in Botswana. It is only about the size of a plump almond.

But to George Harlow, a geologist and curator of the museum’s Halls of Gems and Minerals, it is a thrilling road map through millions of years of history.

“This diamond has an extraordinary story,” he said in a phone interview on Nov. 10, the opening day of the exhibition. “For most of history, we didn’t know why diamonds were blue. They were thought to be low in nitrogen. But, in fact, they contain borons, and as soon as the boron is more abundant than nitrogen, the diamond becomes blue.”

For those of us who have not thought about atomic element No. 5 since high school chemistry class, let’s just say that the mineral can exist deep below the earth’s surface before it razzle-dazzles us in the form of blue diamonds.

While most diamonds are formed about 100 to 150 miles below the earth’s surface, the Okavango Blue probably was formed more than 400 miles below in what is called the transition zone, Dr. Harlow said. And the diamond’s boron comes from seawater, which, over millions of years, has been swished around inside the earth by a process called subduction, in which oceanic tectonic plates slid underneath continental plates. That pushed flecks of boron deeper into the transition zone to begin their long journey to diamond-hood.

“It’s all about plate tectonics,” Dr. Harlow said. “The interpretation now is that the seafloor founders much deeper into the earth’s interior than we previously thought.”

The Okavango Blue — on public display for the first time — will remain at the museum, along with more than 1,000 other rough-cut diamonds from Botswana, for about five more months (the exhibition’s closing date is not yet determined, the museum said). The diamond is on loan from the Okavango company, which is wholly owned by the government of Botswana. Both share their name with the country’s vast inland Okavango Delta, known for its wildlife and unspoiled beauty.

Dr. Harlow noted that the public had a centuries-old fascination with blue diamonds, particularly the 45.52-carat Hope Diamond, which had been traced to 17th century India and now is at the Smithsonian National Museum of Natural History in Washington.

“I think the history of the Hope Diamond has driven the interest because a lot of people don’t realize that diamonds give out spectral colors,” Dr. Harlow said. “When people see the Okavango Blue, they’ll be surprised that they see more than just blue.”

And he hopes that those gawking at the gem’s rainbowlike spectrum might also see what he sees.

“Diamonds are a time capsule because they are old, often two billion years or more,” Dr. Harlow said. “They also teach us a lot about our planet.”

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