Diamonds might be forever but that doesn't stop them being bought and sold. One stone thought to have once belonged to Marie Antoinette, the last queen of France, has just sold for US$14 million (£10 million) at an auction in New York - about three times the asking price. Set into a platinum ring and weighing a total of 15.5 grams, the clue to the diamond's uniqueness is in its name: the Marie-Thérèse pink.
Author
- Elton Santos
Reader in Theoretical and Computational Condensed Matter Physics, University of Edinburgh
This 10.38 carat pink diamond has been changing hands for generations, and previously sold at an auction in Geneva for an unknown amount. Pink diamonds are very rare and there are many things that scientists still don't know about them.
Diamonds are generally formed under intense heat and pressure deep within the Earth's mantle, roughly 150-200 kilometres below the surface. Most natural diamonds crystallise over billions of years, composed almost entirely of carbon atoms arranged in a tightly packed, cube-like structure.
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Coloured diamonds are geological anomalies. Variations include pink, blue, orange, yellow, red, green, brown and black, most of which can be explained by impurities in their crystal lattice. Yellow diamonds contain nitrogen, for example, while blue ones contain boron.
Pink diamonds are not caused by such impurities. Scientists believe that the pink hue arises from a distortion in the diamond's atomic lattice structure. Intense pressure deep underground creates forces (known as shear forces) that twist and compress atomic layers, which alter how the stone reflects light.
It's this "plastic deformation" which results in the pink coloration, reducing the green light in the visible spectrum so that it shifts the overall colour that we see towards pink.
Only a small fraction of diamonds undergo such extreme and precise pressure and temperature conditions during their formation. These factors make them very difficult to be created and even harder to predict where they will be formed. As a result, pink diamonds are the rarest of all coloured diamonds apart from red ones, which are formed by an even more intense version of the same process.
Aussie rules
For decades, the Argyle mine in western Australia was the world's primary source of pink diamonds (and also red ones), producing over 90% of the global supply. The mine is located at a unique geological area by a so-called lamproite volcanic pipe, as opposed to the more common kimberlite pipes found at most other diamond mines. Without getting too much into the technicalities, lamproite pipes tend to be less explosive and have more unusual minerals like leucite and rich potassium.
The Argyle mine is located in the Kimberley region, which experienced intense tectonic activity during the Paleoproterozoic era, over 1.6 billion years ago. This meant that the lamproite pipe was formed under extreme pressures and temperatures.
This is believed to have caused the lattice defects in the diamonds that were pushed to the Earth's surface, which are responsible for their pink and red colours. The deep mantle depths in the mine were also crucial, since this translates into higher internal pressures and temperatures.
Even so, less than 0.1% of the diamonds extracted from Argyle were classified as pink (and only 0.00000002% were red, if you calculate the proportion of red carats found). The mine then closed in 2020 after 37 years of production because its reserves were exhausted, making pink diamonds even more scarce and valuable.
Other known sources include Brazil, India, Russia and South Africa, but these mines yield pink diamonds far less frequently. The rarity of high-quality pink diamonds has made them highly sought-after by collectors and investors alike, as demonstrated by the high sale price of the Marie-Thérèse pink. That diamond was actually pink-purple, with the purple hue caused by hydrogen being absorbed into the atomic structure during the stone's formation, making it rarer still.
Advanced techniques involving shining infrared light and X-rays into the stones - respectively known as infrared spectroscopy and high-resolution X-ray diffraction - have provided scientists with insights into the structural changes that cause pink and red diamonds.
Yet many questions remain unanswered, and the study of pink diamonds continues to be an active area of scientific investigations in mineral physics and crystallography. This has included creating pink diamonds (and other colours such as blues) in the laboratory by replicating the natural processes that form them, but in a more controlled, accelerated way.
These lab-grown pink diamonds look nearly identical to their natural versions to the human eye, but can yet be differentiated through optical techniques. One method is infrared absorption, which detects how the diamond absorbs light and vibrates at specific frequencies.
Another clue is the presence of sharp peaks in the visible light spectrum that indicate certain impurities, like hydrogen or nitrogen, which are often found in natural stones. In the same style as a CSI investigation, these techniques provide the last word in whether a pink diamond is from a mine such as Argyle, a lab-grown pink, or a clear natural diamond that has been treated pink artificially .
Even after years of improving the process for making pink diamonds synthetically, the mechanical distortions responsible for their exotic colour still can't be replicated precisely under laboratory conditions. Scientists don't understand all the atomic processes involved in their colouring becoming permanent to be able to recreate them perfectly.
The same is actually also true for other synthetic diamonds, though they are becoming harder and harder to detect as the technology improves. In short, pink diamonds (and red ones) remain among the most remarkable precious stones in the world. Unless and until that changes, we can keep expecting them to change hands for ridiculous amounts of money.
Elton Santos receives funding from EPSRC, Royal Society, and is affiliated with the Donostia International Physics Center, San Sebastián, Spain.
