金伯利岩的故事·鑽石從地幔到人間的搬運夫The Story of Kimberlite · The Carrier That Brings Diamonds from the Mantle into the Human World
The World of Gemstones · Diamond SeriesThe Story of Kimberlite · The Carrier That Brings Diamonds from the Mantle into the Human World
The World of Gemstones · Diamond Series金伯利岩的故事·鑽石從地幔到人間的搬運夫
寶石世界·鑽石篇

We already know that most gem-quality diamonds form deep within the mantle, approximately 150 to 200 kilometers beneath the Earth’s surface, while a small number of superdeep diamonds may originate in even deeper regions of the mantle. Many diamonds formed hundreds of millions or even billions of years ago and remained preserved for immense spans of time under temperatures exceeding one thousand degrees Celsius and pressures tens of thousands of times greater than those at the Earth’s surface.

How, then, could diamonds formed at such extraordinary depths ever reach the surface of the Earth and ultimately be discovered by humanity?

For this, they depend upon an exceptionally unusual form of deep-source volcanic activity.

This volcanic activity can carry material from the depths of the mantle, including diamonds that had formed long before, toward the surface at astonishing speed. After the magma cools, it forms a distinctive volcanic rock known as kimberlite.

Kimberlite is not the place where diamonds are created. Most diamonds had already formed and remained within the mantle for hundreds of millions or billions of years before the kimberlite magma appeared. Kimberlite’s role is to transport these diamonds from deep within the Earth to locations close to the surface.

Therefore, if we imagine a diamond as a traveler who has slept for ages within the mantle, kimberlite is the carrier that brings it into the human world.

If deep-source magma rises too slowly, diamonds may undergo corrosion or damage as temperature, pressure, and the surrounding chemical environment change, and some may even become partially graphitized. Rapid ascent is therefore one of the most important conditions determining whether diamonds can survive.

The remarkable nature of kimberlite lies in its abundance of carbon dioxide, water, and other volatile substances. It is somewhat like a giant bottle of carbonated drink that has been violently shaken and then suddenly opened: when pressure deep underground is rapidly released, the magma expands explosively, sweeping mantle rocks and diamonds upward in a powerful rush.

We already know that most gem-quality diamonds form deep within the mantle, approximately 150 to 200 kilometers beneath the Earth’s surface, while a small number of superdeep diamonds may originate in even deeper regions of the mantle. Many diamonds formed hundreds of millions or even billions of years ago and remained preserved for immense spans of time under temperatures exceeding one thousand degrees Celsius and pressures tens of thousands of times greater than those at the Earth’s surface.

How, then, could diamonds formed at such extraordinary depths ever reach the surface of the Earth and ultimately be discovered by humanity?

For this, they depend upon an exceptionally unusual form of deep-source volcanic activity.

This volcanic activity can carry material from the depths of the mantle, including diamonds that had formed long before, toward the surface at astonishing speed. After the magma cools, it forms a distinctive volcanic rock known as kimberlite.

Kimberlite is not the place where diamonds are created. Most diamonds had already formed and remained within the mantle for hundreds of millions or billions of years before the kimberlite magma appeared. Kimberlite’s role is to transport these diamonds from deep within the Earth to locations close to the surface.

Therefore, if we imagine a diamond as a traveler who has slept for ages within the mantle, kimberlite is the carrier that brings it into the human world.

If deep-source magma rises too slowly, diamonds may undergo corrosion or damage as temperature, pressure, and the surrounding chemical environment change, and some may even become partially graphitized. Rapid ascent is therefore one of the most important conditions determining whether diamonds can survive.

The remarkable nature of kimberlite lies in its abundance of carbon dioxide, water, and other volatile substances. It is somewhat like a giant bottle of carbonated drink that has been violently shaken and then suddenly opened: when pressure deep underground is rapidly released, the magma expands explosively, sweeping mantle rocks and diamonds upward in a powerful rush.

According to different geological models, kimberlite magma may travel through more than one hundred kilometers of mantle and crust within a matter of hours or perhaps one or two days, carrying diamonds to positions close to the Earth’s surface.

Because the ascent is sufficiently rapid, the diamonds’ original crystal structures are more likely to remain intact rather than being destroyed during a prolonged and gradual journey upward.

This type of deep-source magmatic activity may be extraordinarily rapid and violent, producing pipes, volcanic fragmental rocks, and other complex volcanic structures near the surface.

The most characteristic of these structures is the kimberlite pipe.

Kimberlite pipes usually widen toward the top and narrow downward, resembling an inverted funnel or chimney. They are pipe-shaped bodies left behind as ancient kimberlite magma forced its way through the crust and approached the surface, while the magma itself originated deep within the mantle.

點擊查看大圖
Click to enlarge

Fortunately—or perhaps regrettably—human history contains no reliable record of anyone directly witnessing a typical diamond-bearing kimberlite eruption.

Most known kimberlites are extremely ancient, but the Igwisi Hills volcanoes in Tanzania are thought to be among the youngest known kimberlite volcanoes, possibly having formed only about ten thousand years ago. Even so, that was still too early in human history for anyone to leave a true record of the event.

Scientists have studied the relationship between kimberlite eruptions and plate movements over hundreds of millions of years and have found that kimberlite activity often becomes more frequent when supercontinents begin to break apart and the crust is placed under tension.

Some researchers have even converted the timing of kimberlite eruptions from different geological periods into musical notes, creating a piece intended to represent the “rhythm of activity deep within the Earth.”

The name kimberlite comes from the South African city of Kimberley. It was in this region during the late nineteenth century that humanity first discovered primary rock bodies containing large quantities of diamonds, opening the era of modern diamond mining.

Kimberlite is sometimes described as the diamond’s “express train through time and space.” The comparison is vivid because kimberlite begins its journey deep within the mantle and rapidly carries diamonds that have lain there for hundreds of millions or billions of years toward the surface.

Before the nineteenth century, most diamonds known to humanity were alluvial diamonds—stones discovered in rivers, valleys, streambeds, or gravel deposits. Ancient India, and later Brazil, were both important sources of such alluvial diamonds.

People could find diamonds in rivers, yet they had no idea where those diamonds had originally come from.

It was not until the 1860s that the history of diamonds in South Africa began to change dramatically.

In 1867, a fifteen-year-old South African boy named Erasmus Jacobs found a sparkling stone near the Orange River and initially treated it simply as a toy.

The stone was later confirmed to be a diamond weighing 21.25 carats and was named the Eureka Diamond.

The identification of the Eureka Diamond generated enormous interest in South Africa’s diamond resources. More diamonds were subsequently discovered, including the celebrated Star of South Africa in 1869.

As the discoveries continued, thousands of prospectors and adventurers poured into South Africa, eventually igniting a diamond rush that swept across the region.

At first, people continued searching mainly in rivers, gravel deposits, and the yellow soil near the surface. As digging proceeded deeper, however, they began to encounter a hard blue-gray rock that became known at the time as “Blue Ground.”

Even more astonishingly, this blue-gray rock directly contained natural rough diamonds.

For the first time, humanity realized that diamonds were not merely loose stones scattered through rivers; they also occurred within a special type of primary volcanic rock body.

This rock was later named after the Kimberley region of South Africa and became known as kimberlite.

To extract diamonds from the kimberlite, large numbers of miners used picks, shovels, and simple tools to conduct open-pit excavation near the town of Kimberley.

After years of mining, their efforts produced a gigantic excavation measuring more than 460 meters across at the surface and approximately 240 meters deep during the period of open-pit operation. It later became known as the Big Hole.

It became one of the world’s most famous open mining excavations and one of the most representative historical remains of the South African diamond rush.

Geological research has shown that kimberlite is a type of igneous rock originating deep within the mantle and rich in volatile components. It is capable of rising rapidly over a short period of time, carrying mantle rock fragments, minerals, and diamonds toward the near-surface environment.

This ability to transport material so rapidly is the key to preserving diamonds and ultimately making their discovery by humanity possible.

However, not all kimberlite contains diamonds.

Even when a particular kimberlite body does contain diamonds, it may not possess enough of them, or diamonds of sufficient quality, to justify commercial mining. Kimberlite pipes capable of supporting large diamond mines represent only a tiny fraction of those found in nature.

Consequently, the discovery of kimberlite merely indicates that diamonds may be present nearby; it does not guarantee the existence of a commercially valuable deposit.

This uncertainty ensures that diamond exploration and mining remain filled with both danger and temptation.

To locate kimberlite pipes concealed underground, geologists search not only for diamonds themselves but also for certain distinctive indicator minerals.

Common kimberlite indicator minerals include particular varieties of chromium-rich pyrope garnet, chrome diopside, magnesium-rich ilmenite, and forsteritic olivine.

These minerals can survive prolonged weathering and transportation and may remain preserved in river sediments. To geologists, they are like clues scattered across the land, helping them determine whether kimberlite pipes and diamonds may exist upstream or beneath the ground.

The discovery of South African kimberlite formally opened the age of modern primary diamond mining.

Humanity no longer depended solely upon chance discoveries along riverbanks, but began to use geology, mineralogy, and engineering technology to search deliberately for primary diamond deposits.

Afterward, more kimberlite pipes were discovered throughout the world. Large modern diamond mines were established in South Africa, Botswana, Russia, Canada, and other regions.

These mines not only produced astonishing quantities of diamonds but also yielded many of the most legendary diamonds known to the world.

The Cullinan Diamond, which we mentioned earlier, was discovered in a South African mine developed in a kimberlite pipe.

It did not come from the Kimberley Big Hole, but from the mine then known as the Premier Mine, later renamed the Cullinan Mine.

In 1905, this enormous gem-quality rough diamond, weighing 3,106.75 carats, was discovered and astonished the entire world.

How the Cullinan Diamond was cut, how it entered the British royal collection, and the place it came to occupy in history will be discussed in detail later in our series on famous diamonds.

The significance of kimberlite does not lie solely in the possibility that it may contain diamonds.

It is also like a letter sent from the depths of the Earth.

During its rapid ascent, kimberlite magma carries fragments of mantle rock and minerals toward the surface. By studying these materials, scientists can gain knowledge about mantle environments that humans cannot reach directly and infer the temperatures, pressures, composition, and evolutionary history of the deep Earth.

Kimberlite therefore possesses not only major economic value but also exceptional scientific value.

To an ordinary reader, kimberlite may seem no more than an unfamiliar and difficult name for a rock.

In reality, however, it is one of the most important bridges between diamonds and the human world.

Without this rapid and violent form of deep-source magmatic activity, vast numbers of diamonds formed in the mantle might have remained buried underground forever, entirely beyond human sight.

Perhaps we can understand it this way: diamonds were discovered by humanity not simply because they existed, but because the Earth prepared a road leading them toward the surface and provided them with a faithful and powerful carrier.

That road is the kimberlite pipe left behind by ancient deep-source magma.

And the carrier that brings diamonds from the depths of the mantle into the human world is kimberlite.

(To be continued)