Cullinan Diamond Reveals Earths Secrets

From Mantle to Monarchy: How a Giant Diamond Unlocked Our Planet's Secrets

The magnificent diamonds sparkling in King Charles III’s coronation regalia possess a story that travels far deeper than royal history. These gems are not merely symbols of wealth and power; they are scientific envoys from the heart of the Earth. The colossal Cullinan diamond, the centrepiece of the Crown Jewels, provides a unique window into the planet’s hidden interior. Geologists now understand that these exceptional stones are fragments of a lost world, carrying secrets of planetary formation that are rewriting our understanding of the ground beneath our feet.

The Discovery of a Behemoth

The story began on 26 January 1905, at the Premier No. 2 mine in South Africa. Frederick Wells, the mine’s surface manager, was conducting a routine inspection 5.5 metres underground when a gleam of light from the rock wall caught his eye. Initially dismissing it as a piece of glass planted by miners as a prank, he decided to investigate. His pocketknife promptly snapped against the object’s unyielding surface. After careful extraction, the find was revealed to be a diamond of unbelievable proportions. The stone weighed an astonishing 3,106.75 carats and was almost the size of a human fist, displaying a remarkable blue-white colour and exceptional clarity.

A Perilous Journey to London

News of the discovery, named the Cullinan after the mine’s owner, Sir Thomas Cullinan, created a global sensation. However, it also presented a formidable security challenge: transporting the world’s most valuable stone 12,755 kilometres to London to find a buyer. A clever piece of misdirection was arranged. A replica of the diamond was conspicuously placed in the captain’s safe aboard a steamboat, guarded by police detectives to attract any would-be thieves. Meanwhile, the real Cullinan, wrapped in a plain cardboard box, was sent to London by ordinary registered post, arriving without incident.

A Gift for a King

Despite immense interest, the colossal diamond remained unsold for two years. In 1907, the Transvaal Colony government, then under British rule, purchased the diamond for £150,000. Prime Minister Louis Botha proposed presenting the gem to King Edward VII as a gesture of loyalty following the Second Boer War. The king accepted the gift, which was formally presented on his 66th birthday. This act secured the diamond's future, ensuring its preservation as part of the historic jewels of the British Crown.

From Rough Stone to Royal Splendour

King Edward VII entrusted the complex task of cutting the Cullinan to Joseph Asscher & Co. of Amsterdam, the most renowned diamond cutters of the era. Joseph Asscher studied the stone for months, planning the first crucial cleavage. His first attempt famously broke the blade. Ultimately, the process was a triumph, yielding nine major stones, numbered Cullinan I to IX, and 96 smaller brilliants. The largest, the 530.2-carat Cullinan I, became known as the Great Star of Africa. The second-largest, the 317.4-carat Cullinan II, is the Second Star of Africa.

A Coronation Showcase

The Cullinan’s descendants are the undisputed stars of the Crown Jewels of Britain. During King Charles III’s coronation, the Cullinan I shone from its setting in the Sovereign’s Sceptre with Cross, which had to be reinforced to hold its weight. The Imperial State Crown prominently features the Cullinan II, worn by the monarch after the ceremony. Queen Camilla wore the modified Queen Mary’s Crown, which was specially set with Cullinan III, IV, and V gems from Queen Elizabeth II’s personal collection.

More Than Just a Gemstone

While celebrated for their scale and purity, the Cullinan and similar gems belong to a scientifically distinct category of diamonds known as "CLIPPIR". This acronym describes their key characteristics: Cullinan-like, Large, Inclusion-Poor, Pure, Irregular, and Resorbed. These are not just aesthetic qualities. They are geological puzzles presented as jewellery, pointing to an origin story far different from that of common diamonds, one that begins in the planet's deep mantle. These stones offer insights into an otherwise inaccessible realm.

The Scientific Gatekeepers

Studying such valuable gems presents immense challenges. Researchers rarely gain access to large, high-value diamonds, which are tightly controlled by the market. Maya Kopylova, who is a professor at the University of British Columbia, notes that many samples accessible for study are stones that would otherwise be discarded. However, researchers working at the Gemological Institute of America (GIA) are uniquely positioned. Because a vast number of gems pass through the GIA for valuation, scientists like Dr Evan Smith have unprecedented access to a huge collection of the planet's most exceptional diamonds.

Image Credit - by Parent Géry, CC BY-SA 3.0, via Wikimedia Commons

A Glimpse into the Forbidden Zone

This unique access led to a revolutionary discovery. Dr Smith and a global research team examined 53 of the biggest and most transparent diamonds available. Their analysis revealed that almost 75 percent of these CLIPPIR gems held miniscule metallic inclusions which had resisted rusting—something not found in ordinary diamonds. The remaining stones contained a type of garnet known to form only under the incredible pressures within the Earth's mantle. Together, these chemical hitchhikers provided the first concrete evidence of a super-deep origin.

The Super-Deep Origin

Common diamonds form at depths of around 150 to 200 kilometres, in the rigid base of continental plates. The inclusions in CLIPPIR diamonds, however, indicate they formed much deeper, in a "Goldilocks zone" between 360 and 750 kilometres beneath the crust. This region, a place called the mantle's transition zone, is deep enough for the metallic inclusions to remain unoxidised but shallow enough that garnet minerals would not break down. The finding confirmed that the world's most magnificent gem diamonds are also the planet's deepest.

A Messenger from the Planet's Lower Mantle

Further research by Dr Smith and his colleague Dr Wuyi Wang on a 124-carat diamond provided even more startling proof. Within this gem, they found the chemical remnants of a mineral called bridgmanite. Bridgmanite is believed to be the most widespread substance on Earth, but it is only stable under the extreme pressures within the lower mantle, more than 660 kilometres deep. Because it breaks down as pressure decreases, it cannot exist on the Earth's surface. Finding its signature trapped inside a diamond was undeniable proof that the diamond had travelled from this incredibly deep and mysterious layer of the planet.

An Alien Mineral on Earth

The existence of bridgmanite was theorised for decades before it was first officially observed in 2014. It was not found in a terrestrial rock, but within a 4.5-billion-year-old meteorite that struck Australia in 1879. The cataclysmic impact that broke the meteorite from its parent asteroid generated pressures that resemble those in Earth's lower mantle, creating the mineral. Its discovery inside a diamond was a monumental moment for geology, confirming that these gems are direct samples from a part of our world we can never otherwise reach.

An Alternative Genesis

This deep origin explains the unique qualities of CLIPPIR diamonds. While most diamonds are thought to crystallise from carbon-rich fluids derived from ancient seawater pulled deep underground, super-deep diamonds have a different beginning. They form from carbon that has dissolved inside liquid metal, a molten iron-nickel alloy deep inside the mantle. This metallic fluid contains very little nitrogen, which is why CLIPPIR diamonds are almost always exceptionally pure and transparent, classifying them as rare diamonds of the Type II classification.

Confirming a Planet-Sized Process

The implications of this discovery extend to the very foundations of geology: plate tectonics. For decades, scientists have theorised that as tectonic plates shift, heavier oceanic plates are forced to slide beneath continental plates in a process called subduction, eventually sinking toward the lower mantle. While seismic imaging could track the plates descending, there was no physical proof of their ultimate fate. Super-deep diamonds, forged from materials carried down by these plates, provide that long-sought physical evidence.

Recycling the Ocean Floor

The chemical signatures within super-deep diamonds tell a remarkable story of global recycling. Inclusions within the gems contain minerals and isotopic signatures that directly match materials found in oceanic crust, which has been altered by seawater. This confirms these diamonds are fundamentally fragments from the old seafloor, subducted hundreds of kilometres deep, transformed under immense pressure, and then brought back to the surface. They are physical proof of a planet-wide convection system that has shaped Earth for billions of years.

The Mantle's Hidden Oceans

These deep-Earth messengers carry clues about more than just rock. In 2021, analysis of an unusual diamond from Brazil revealed an inclusion of a striking blue mineral called hydrous ringwoodite. Ringwoodite is a form created under high pressure of olivine, the main component of the planet's upper mantle, but this specific type contains as much as 2.5% water. The discovery provided the strongest evidence yet that vast quantities of water, potentially equivalent to all the world's oceans, are stored deep within the mantle’s transition zone.

Image Credit - by aiva., CC BY 2.0, via Wikimedia Commons

Solving the Water Mystery

The origin of Earth’s surface water has long been a subject of debate. Scientists believed it ultimately originated within the mantle, but the storage mechanism was unclear. Finding water-rich ringwoodite inside a diamond provides a plausible answer. It suggests that a deep water cycle is an integral part of our planet's geology, with subducting oceanic plates transporting water deep inside the mantle, where it is stored in high-pressure minerals before potentially returning toward the surface via volcanic activity.

An Unlikely Scientific Tool

There is a strange irony in the fact that humanity's fascination with sparkling jewellery has driven the science forward. The multi-billion-pound diamond industry, with its complex and often destructive history, has unintentionally funded the recovery of the most valuable geological samples on our world. Without the commercial quest for these gems, the messengers from the deep Earth would have stayed undiscovered, and our planet's inner workings would remain largely a matter of theory.

The Duality of Value

This connection creates a peculiar tension between a diamond's monetary and scientific worth. The immense financial value of a large, flawless diamond makes it accessible for non-destructive study at institutions like the GIA. However, that same value means scientists can never perform more detailed, destructive analysis. Dr Smith has noted the frustration of holding an intriguing scientific specimen worth millions, knowing it could unlock more secrets if only it might be broken open for deeper study.

Beauty in its Natural State

While polished diamonds are prized for their fire and brilliance, their natural, uncut forms tell a different story. The rough stones that are extracted from the ground are often uneven and rough, their surfaces intricately etched by their violent, high-speed ascent to the surface in kimberlite magma. These natural sculptures are a direct record of the immense geological forces that shaped them across millions of years. This raw, unpolished state holds a unique beauty—a physical history of an epic journey from deep within our planet.