The Power of Knowledge

The largest diamonds in the British Crown Jewels may be pieces of the ancient ocean floor, which have drifted down into the interior of our planet – then come back up again.

The package arrived in a plain cardboard box. It was simply addressed to S Neumann & Co – a mining sales agency in the centre of London – and weighed just over a pound (around 500g). But this was no ordinary cargo.It was April 1905, and three months earlier, the surface manager at the Premier Mine in South Africa had been completing a routine inspection 18ft (5.4m) underground, when he glimpsed a reflected light in the rough wall above him. He assumed it was a large piece of glass hammered in by colleagues as a practical joke. Just in case, out came his pocket knife, and after some digging… the knife promptly snapped. Eventually the rock was removed successfully, and revealed to be a bona fide diamond – a monster 3,106.75-carat stone, almost the size of a fist. It was not only enormous, but unusually transparent.The Cullinan, as it became known, is the largest diamond ever found. Once it had been polished and cleaved into several more manageable stones, the largest crystal it yielded would shine like the cool glow of a star in a distant galaxy. As a result, this stone – the Cullinan I – is sometimes known as the Great Star of Africa.

Nearly 120 years later, the mega-diamond has not been forgotten. During the late Queen Elizabeth II’s funeral procession, several of the Cullinan’s descendants were placed on the Monarch’s coffin, and rode along with her – they were only removed as she was lowered into the royal vault. That’s because today these giant gems are part of the Crown Jewels, normally kept in the Tower of London and brought out for state events – the Cullinan I now resides in the British Sovereign’s Sceptre, while its next-largest sibling, the Cullinan II, is embedded in the Imperial State Crown.

However, before the rough diamond could have its makeover and take its place in history, it needed to be sold – and London was chosen as the most promising location to do this. This presented a problem: how do you transport such a valuable stone 7,926 miles (12,755km), without it being stolen?

In the end, the precious rock was sent all the way from Johannesburg by ordinary registered post, at a cost of just three shillings or about 75 US Cents at the time (around £11.79 or US$13.79 today). Meanwhile, a replica of the diamond made the long voyage to London by steamboat – it was placed conspicuously within the captain’s safe and guarded by police detectives as a decoy. Amazingly, both made it to their destination. After years of failing to sell, the diamond – the real version, this time – was purchased by the Transvaal government for £150,000 (£20m or US$22.5m today) and gifted to King Edward VII. But though they’re renowned across the globe for their size and transparency, these characteristics are no accident. The Cullinan was a “Clippir” diamond – a member of a special category of the very largest and clearest examples ever found. And there is more to them than meets the eye.For all their beauty, diamonds are really fragments of the deep Earth – intriguing geological anomalies disguised as mere jewellery. These strange rocks are capsules from another world – a mysterious realm of unfathomable pressures, swirling green rock, and elusive minerals, far below the Earth’s surface. Scientists around the globe have been studying them for decades to reveal the region’s secrets – and intriguingly, it’s the very diamonds that we value most that have the most unusual stories to tell. In fact, large rocks like the Cullinan are transforming our understanding of the inside of our planet.An unusual opportunity Sitting in front of a microscope at the Gemological Institute of America (GIA) in 2020, Evan Smith carefully stretched some rubber gloves over his fingers, and peered into the instrument’s lenses. Beneath was a diamond worth almost as much as a small country – about the size of a walnut, with 124 carats of wonderous brilliance. To reach this point, Smith had already navigated almost military levels of security – iris scans and identity checks, followed by layer after layer of locked doors, secure lifts and mysterious restricted corridors. While he worked, video cameras streamed a constant view of the room to watchful security guards.

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 analysed 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.

From the depths

Back in the 1980s, geologists began to notice that some diamonds looked different to others – sometimes they contained minerals that could suggest they formed at higher pressures than regular ones (more on this later). “We started to wonder whether some diamonds might actually be formed deeper than others.”

THE CULLINAN DIAMONDS Once it had arrived in London safely, the rough Cullinan diamond was sent to be cut by Joseph Asscher. It’s reported that the rock was so massive, the first heavy blow of the hammer led to yet another knife casualty (it broke) – and made him faint. However, eventually Asscher managed to divide the diamond into nine major stones, the largest of which was 530.20 carats – a measure of its weight – and 96 smaller ones. While the larger stones became part of the British Crown Jewels or the British monarch’s private collection, the smaller fragments were sold to various clients around the world.

Around the same time, they noticed a puzzling pattern. Most diamonds – called Type I – contain a significant amount of nitrogen, which affects their crystal structure and can add a hint of pale yellow or brown. Occasionally, though, a diamond has almost no detectable traces of this element. These are the Type II diamonds, and the phenomenon is extremely rare – except in the very largest diamonds.”It’s not just that they’re big that sets them apart,” says Smith. “When you look at these big, high-quality [type II] diamonds, like the Cullinan, there turned out to be something strange about them, that makes them more likely to fall into this category that should otherwise be something very rare. So this was kind of a long-standing mystery.”Eventually scientists discovered that some diamonds are “super-deep”, and identified a handful of mines where they were most likely to be found – including the Cullinan mine in South Africa and Letseng mine in the nearby kingdom of Lesotho, where Smith’s 124-carat specimen originated.