Usually, when goods are faulty, we expect their value to drop, but the exact opposite is true for diamonds. Ironically, it’s imperfections that give diamonds color – and these “fancy” diamonds are some of the most sought after in the world.
Diamonds are made up of carbon atoms organized into compact structures. Clear, perfect diamonds sparkle because light reflects off their inner surfaces. Of course, these diamonds are precious.
However, when diamonds contain impurities or are subjected to intense pressure, they can develop distinctive colors. Colored diamonds are highly prized for their beauty and rarity, and can be several times more expensive than clear diamonds.
It is therefore not surprising that the world was astonished when the Australian company Lucapa Diamond Company announced the discovery of the Lulo Rose this week. The 170-carat rough pink diamond, found in Angola, is the second largest pink diamond ever discovered.
Colored diamonds represent only 0.01% (one in 10,000) of diamonds mined worldwide. Natural yellow and brown are the most common and, as you would expect, therefore not too expensive.
However, blue, green, purple, orange, pink and red diamonds are extremely rare and exist in minute quantities. These are really coveted.
Ultra-rare colored diamonds were sold at record prices. The Pink Star, weighing 59.6 carats (about the size of a strawberry), is the most expensive diamond ever sold, selling for a staggering A$94.2 million.
It should be mentioned that the pink star originally came from a diamond weighing 132.5 carats. More than half of its weight was lost in the process of cutting and polishing the stone – a process that took 20 months.
At 170 carats, it is entirely possible that the Lulo Rose, if sold at auction, could become the most expensive diamond in history.
The only pink diamond bigger than it is the Daria-i-Noor (185 carats), which is the centerpiece of Iran’s Crown Jewels, and has never been offered for sale.
So why are colored diamonds so rare?
Physical and chemical purity results in clear diamonds. Thus, colored diamonds form as a result of imperfections. But it is very rare for imperfections to appear in a material that is not only extremely hard, but also chemically simple.
There are three main imperfections that produce colored diamonds: impurities, damage and distortion. These are imperfections in the diamond’s structure that affect how light passes through the gem – specifically the diffraction and absorption of different wavelengths of light. And that’s what leads to the different colors that we see.
The main impurities in diamonds is in the form of very light elements, such as nitrogen, boron and hydrogen, which are usually found in abundance in the oceans and atmosphere. These elements can give rise to specific colors. For example, boron-rich diamonds will be blue, while nitrogen-rich diamonds will be yellow.
Then there is damaged diamonds, where damage occurs when a diamond has been sitting next to radioactive elements, such as uranium, thorium or potassium.
To finish, Distortion refers to the twisting and bending of a diamond’s crystal lattice under immense pressure. This causes flaws a hundred times smaller than the width of a human hair, but it is enough to diffract light and bring color to the gem.
Every colored diamond has a cocktail of imperfections, which is why no two diamonds are the same. However, they all have one thing in common: every diamond is rooted in geological history. Diamonds can be billions of years old. Meanwhile, some have traveled from the depths of the planet to its surface, only for us to claim them.
Take yellow and blue diamonds. Light elements such as nitrogen and boron are concentrated in our oceans and atmosphere, but we know that diamonds must form in the heart of the planet. So the key here is plate tectonics, and more specifically a process called subduction.
Subduction is a geological process by which oceanic lithosphere (part of the outer crust) is recycled into the Earth’s mantle. This is how the light elements manage to penetrate deep inside the Earth – eventually becoming part of the colored diamonds.
A unique window on the interior of the Earth
Pink diamonds have their own geological history. Most scientists believe that the carbon on our seabed, when pushed back inside the Earth, can not only form diamonds, but also deform.
When subjected to enormous pressure and considerable temperatures, the way the carbon is supposed to be put together is distorted, and this is what leads to the formation of pink diamonds.
However, if the Earth pushes a little too hard, the pink hue quickly turns brown or, as some would call it, “champagne” or “cognac.” Yet there is still a lot we don’t know about pink diamonds. For one thing, why do around 80% of pink diamonds come from just one recently closed mine in Western Australia?
The Argyle Mine was once the largest diamond mine in the world, but close in 2020 after becoming economically unviable. Yet this mine is truly unique, not only because of the number of pink diamonds it has produced, but also because it is located in a geologically intriguing region.
For years, scientists and diamond companies believed that diamonds large enough to mine could only be found in the heart of ancient continents. But the Argyle Mine sits at what was once the edge of two continents that collided and came together just 1.8 billion years ago.
It may seem long, but in geological terms, it is not. Argyle and its pink diamonds likely hold the answer to pink diamond formation, but finding it will require further investigation.
With the mine having closed and pink diamonds becoming increasingly rare, one can only hope that scientists will soon unravel the mystery of how pink diamonds form. Maybe with this knowledge we could find yet another treasure.
Luc DoucetResearcher at the Earth Dynamics Research Group, member of TIGeR, Curtin University; Denis FougerouseResearcher, School of Earth and Planetary Sciences and Geosciences Research Institute (TIGeR), Curtin Universityand Hugo Olierookgeology researcher, Curtin University
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