Diamonds: in stars, Earth’s crust, and the lab

Diamonds form under extreme pressure and temperature conditions. People are probably most familiar with mined diamonds from the Earth's crust, precious, glittery stones frequently used in jewellery. They are of course beautiful, and scarce, which is why people pay so much for them. They are also pretty interesting to study. Some of my friends got engaged recently, which inspired me to explore the science behind these tiny shimmering objects frequently encrusted into rings.

What is a diamond?

Diamonds are crystals made of pure carbon. Interestingly, this is the same element that makes up graphite, which could not be more different as a material. Graphite is the stuff pencil lead is made from- common, cheap, grey and soft. Diamonds however are one of the hardest known substances, famously so hard they can only be scratched by another diamond.

The difference between these materials lies in the arrangement of their carbon atoms. Graphite forms in plane sheets which slide easily past each other. Meanwhile in diamonds, they form a rigid 3D tetrahedral structure, as shown in the sketch below.

Diagram of the atomic structure diamond and graphite, both made of carbon. Credit: Dr Stan Fowler/Youtube

Carbon atoms like to bond to 4 other carbon atoms. In diamond, each carbon atom is bonded to 4 other carbon atoms, meaning there are no free electrons. In graphite however, each carbon atom is bonded only to 3 other carbon atoms, leaving one electron of every atom free to zoom around the atomic structure- hence graphite can conduct electricity while diamond cannot. Their structures are also responsible for their different physical properties- the covalent bonds in diamonds are responsible for their famous strength, as these bonds are extremely difficult to break, while the layers of graphite can easily slide over each other. Famously strong diamonds have other uses beyond jewellery, such as in cutting and drilling equipment.

Diamonds on Earth (and Neptune and Uranus)

Apparently, many people believe diamonds are formed from coal. While this would make a cool story worthy of a fairytale, it is sadly scientifically inaccurate, which can be quickly deduced from the fact that diamonds started forming before the Earth had land plants, the very source from which coal is made. Diamonds are however made from crystallised carbon. Four processes are thought to be responsible for the majority of diamonds found on or near the Earth's surface, with one process in particular accounting for almost all diamonds that have ever been mined.

Geologists believe that these diamonds were formed in Earth's mantle and delivered to the surface by deep volcanic eruptions, as seen in the illustration below.

An illustration of deep source volcanic eruptions bringing diamonds to the surface. Image credit: Geology.com

The formation of natural diamonds requires very high temperatures (900-1400 Celsius) and pressures (273 000 times atmospheric pressure), conditions thought to occur in limited zones in the Earth's mantle around 150km below the surface, in an area called the diamond stability zone. Different pressure and temperature combinations will change the properties of the resulting diamond somewhat. The exact environment needed to create diamonds is not present globally, instead they are thought to be concentrated around the stable interiors of continental plates. Diamonds formed in these regions are then delivered to the surface by deep-source volcanic eruptions. Pieces of the mantle are torn out and carried up rapidly, as shown in the diagram above. The carbon source for these mantle diamonds is thought to be carbon trapped in Earth's interior at the time of the planet's formation or delivered to great depths by subduction. Diamonds can also be found deep inside the atmospheres of ice giants Neptune and Uranus, where carbon can be compressed to the extreme pressures and temperatures needed to form diamonds. These then sink down to the planetary cores in the form of 'diamond rain', a spectacular sight indeed!

Below is a photo of diamonds fresh from the ground, before they are cleaned and cut into the intricate shapes we see on jewellery. They have an interesting history too. The very first diamonds date back to 2500BC in India, according to the Cape Town Diamond Museum. Unlike modern diamonds which are mined, these were simply collected from sediment in rivers and streams. Their beauty has long made these gems desirable, and by the 4th century BC they were traded around the world, affordable only to the very wealthiest on account of their extreme rarity, becoming the ultimate status symbol. According to the museum, diamonds became more accessible around the 19th century when extensive diamond deposits were first found in South Africa. Interestingly, diamonds are actually less scarce than other gemstones, such as rubies and sapphires. However, their production costs include a long processing chain reaching from mining to polishing. It should be noted that diamond mining can cause extensive environmental damage, and workers can be exposed to harsh working conditions for little pay. It is however possible for shoppers to source natural diamonds ethically, although this may cost more. However, I was intrigued to stumble across another alternative.

What diamonds look like before they are cut and processed. Image credit: Livescience/GETTY images

In the lab

One memorable physics lecturer at Exeter taught us a module on condensed matter, which consisted of many diagrams of atomic lattice structures, electrons whizzing around and an entertaining occasion when he jumped on our desks to provide an illustration of this. At one point, he mentioned it was possible to grow diamonds in a lab. I was intrigued. He explained to us that some 'natural' diamond companies were spooked by this, since increasing the supply of diamonds would change the delicate supply/demand balance, making them far less valuable hence reducing their prices. He went on to tell us that some companies went so far as to campaign that lab grown diamonds were not proper, and that only those formed naturally were 'the real deal'. Looking around online, it seems that lab grown diamonds are ~60-85% cheaper than naturally formed ones. The idea of a diamond grown in a lab may seem less romantic than one formed over billions of years in the Earth's crust, but I for one prefer it, as it eliminates some of the darker sides of the diamond trade, such as the environmental damage caused by mining and the exploitation of workers in mines. In truth, they are just as beautiful and sparkly, with only the trained jeweler able to distinguish the two. Sure, they may not have the history of natural diamonds formed over immense timescales, but they tell their own story- one of scientific and technological innovation so advanced we can now replicate a process in nature which takes billions of years and compress the process into a mere week or two!

So, how does this process actually work? Pretty much the same as it does naturally, with high pressures and temperatures created artificially. There are two methods, the first is called the high pressure, high temperature (HPHT) technique, which attempts to mimic the natural process as closely as possible. The second, is the chemical vapour deposition (CVD), which requires less extreme temperatures and pressures. This process can be briefly explained in the following steps:

1. A thin slice of diamond, called a diamond seed is placed in a sealed chamber. The original seed will typically be taken from a diamond created with the HPHT method.

2. The chamber is heated to around 800°C and flooded with carbon rich gas

3. The carbon rich gas ionises under the intense heat, breaking down into pure carbon

4. These pure carbon molecules attach to the original diamond seed, growing as more layers of carbon are added

5. This process continues until a fully formed rough diamond is created

I for one would feel no shame in wearing a lab grown diamond, if anything, I would be proud. If I ever am in a situation where I am buying a diamond, I will be on the lookout for a lab one. In the immortal words of Sir Humphrey from the BBC's classic series Yes Minister, 'times change and we change with the times'. Or for any Latin scholars reading this, Tempora mutantur.

Diamonds in stars

Rihanna is not usually considered a go-to source for scientific learning, but her lyrics 'diamonds in the sky' is not technically just a romantic metaphor, it is also scientifically accurate, more or less. Again, this was something I learnt at Exeter in another physics lecture about stars from birth to death, with a shoutout to another great lecturer Pablo! White dwarfs, which I wrote about in my previous post are the burnt out remnants of stars with similar mass to our Sun. They are made from metallic oxygen and carbon. It is thought that the vast majority, around 97% of stars in our galaxy, will undergo this evolutionary stage. Potentially, their cores could crystallise into huge diamonds, ones that would make even Rihanna's jaw drop.

Stars stay alive by burning their fuel, which means the hydrogen in their cores fuses to make helium and increasingly heavy elements, which releases energy and generates an outward pressure, which counteracts the inwards pressure of gravity, preventing the star collapsing in on itself. Once the fuel runs out however, a process which can take millions to billions of years, the collapse begins. This chaotic process involves the star losing mass and shedding outer layers into space, creating stunning nebulae such as the one shown below. The white dwarf left behind is so dense that it contains as much mass as the Sun in a volume the size of the Earth. White dwarfs have a density of around 1 million kg per cubic metre, while diamonds are around 3500 kg per cubic metre. Fusion no longer occurs here, but they still radiate thermal energy and light up the gaseous shells surrounding them, illuminating the nebulae. Theoretically, white dwarfs can live for billions of years.

The Southern Ring nebula, featuring a white dwarf star in the centre. Image credit: NASA/ESA/CSA/STScI

White dwarf cores are initially liquid, and the star's cooling over time causes the liquid to become solid. The diamond phase occurs when atoms have cooled down enough to form a huge lump of cold, solid, crystallised carbon. Unfortunately, it is thought this process of giant diamond making would take around a quadrillion years (a million billion) and our universe is a mere ~13 billion years old. However, astrophysicists believe a star has been found in the early stages of this transition, the 4.2 billion year old star HD 190412 with a toasty temperature of 6300 Celsius, around the range needed to crystallise a white dwarf.

During crystallisation, the carbon and oxygen atoms inside the white dwarf stop moving around freely and form bonds, arranging into a crystal lattice. This process releases heat, slowing the white dwarf's cooling process. This in turn makes the star appear younger than it actually is. In a funny way, while people may use sparkly cufflinks or jewellery to add a youthful spark, the same effect is achieved in white dwarf stars.

As mentioned earlier, the vast majority of stars in the galaxy eventually form white dwarfs, which means in a time very very far from now, the Milky Way could be full of huge, shining diamonds, glittering away peacefully into the silent darkness.

Nanodiamonds of dust

And finally, for those who prefer their diamonds less bling bling than an entire stellar core, there are little diamonds in space too, nanoscopic particles of crystalline carbon, also known as diamond dust! These were found through a faint glow seen in the Milky Way called anomalous microwave emission (AME), thought to be produced by tiny nanodiamonds which are hundreds of thousands of times smaller than a grain of sand. Nanodiamonds aren't particularly rare in space, which makes sense when one considers carbon is the 4th most abundant element in the galaxy. Nanodiamonds are found inside meteorites - solid pieces of debris from rocky asteroids or comets. It is not known exactly how nanodiamonds form, though possible explanations include cosmic explosions and supernova shockwaves. Another option could be that diamond particles form within protoplanetary discs as illustrated in the drawing below (see also this previous blog post) from a superheated vapour of carbon atoms. Indeed, this is similar to the chemical vapour deposition method used for creating diamonds in the lab.

Artist’s impression of nanodiamonds in a protoplanetary disc around a young star. It is thought these tiny grains of carbon could be responsible for anomalous microwave emission. Credit: S. Dagnello, NRAO/AUI/NSF

Nanodiamonds could help with studying the early universe too. Some models suggest the universe expanded faster than the speed of light after the Big Bang, which would have left an unusual polarisation in the cosmic microwave background (the leftover radiation from when the universe cooled as it rapidly expanded). While this polarisation has not yet been detected, spinning nanodiamonds could be weakly polarised, hence further study of these dusty jewels may reveal some insights into the early universe.

So there we have it, from cores of stars and their dusty surroundings to the crust of Earth and even the lab, diamonds continue to fascinate. And next time you see one of these glittering jewels, take a moment to appreciate the scientific wonder behind their ethereal beauty.