The “super-deep” royal diamonds revealing Earth’s secrets

Smith – a senior research scientist at GIA – was examining the diamond for inclusions, chemical hitchhikers from the interior of our planet that can reveal how the crystal formed, and under what conditions. But working with high-value diamonds is a tricky business – ordinarily, it’s impossible for researchers to get their hands on the largest specimens. They’re sometimes flown around the world to visit potential customers – alas, never scientists.

Maya Kopylova, a professor of mineral exploration at the University of British Columbia, says getting samples of any diamonds is hard, and most of the diamonds she works with would have otherwise been thrown away. “Researchers have to have a good relationship with companies and they will never give you valuable samples,” she says. “So, they will never give us diamonds that are 6mm (0.2 inches) in size or larger.”

Even then, acquiring them is convoluted and expensive – first, Kopylova has to visit the high-security facilities where diamonds are sorted and identify the specimens she’d like to study. Once the acquisition has been approved, then comes the paperwork – all diamonds must travel with a Kimberley Process certificate, which proves its provenance and helps to prevent conflict or “blood” diamonds from entering the market.

However, Smith’s situation is different. At GIA, he has access to one of the largest collections of diamonds on the planet – millions of gems that have been sent there to be valued, so that they can be insured or sold. “If you want to see something rare and unusual, this is the perfect place to go because there are diamonds coming through here all the time,” says Smith. “Every few days, you might get to borrow a diamond for maybe a few hours, maybe a day or two and study it.”

A few years earlier, this is exactly what Smith had done. Together with an international team of scientists, he casually requisitioned 53 of the largest, clearest and most expensive available – including some from the same mine as the Cullinan diamond – and took them back to his laboratory to view under a microscope.

What Smith found was revolutionary. Nearly three-quarters of the Clippir diamonds contained tiny pockets, or “inclusions” of metal that had avoided rusting – not something you’d find in ordinary ones – while the remaining 15 contained a kind of garnet which only forms within the Earth’s mantle, the layer above its molten core.

Together, these inclusions provide chemical clues that the diamonds could only have formed no fewer than 360km (224 miles) and no more than 750km (466 miles) underfoot. In this Goldilocks zone, it’s deep enough to explain the metal inclusions that hadn’t been exposed to oxygen, which is abundant higher up, and it’s not so deep that the garnet rocks would have broken down under the immense pressures of the lower mantle. Ordinary diamonds, meanwhile, originate below the crust, just 150-200km (93-124 miles) down.

For his 2020 study – together with Wuyi Wang, who is vice presedent of research & development at GIA – Smith analyzed the 124-carat diamond and found that it formed at the deeper end of the possible range – at least 660km (410 miles) below the Earth’s surface.

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