How are Diamonds Made?

A diamond is formed when carbon gets squeezed under high pressure and temperature

Table of Contents

The Hope Diamond was mined in the Kollur mines of India. Currently, it is in the possession of the Smithsonian National Museum of Natural History
The Hope Diamond was mined in the Kollur mines of India. Currently, it is in the possession of the Smithsonian National Museum of Natural History, Credit:Wikimedia/David Bjorgen

Diamonds are billions of years old. High pressure and temperature in the upper mantle of the Earth squeezed carbon atoms into diamonds. However, what about diamonds that form in space? And during earthquakes? Indeed, there is a lot to discover.

How are diamonds made?

In the upper mantle

Diamonds form almost 95 to 125 miles (150 to 200 kilometers) below the surface of the Earth in the upper mantle. It has a temperature of 2,100°F (1,150°C) and pressure 50,000 times greater than atmospheric pressure. As a result, the carbon that exists as graphite there forms diamonds. Graphite is the black stuff in pencil lead. Imagine that dull substance being pressed into a pretty gem. Six carbon atoms arranged in a hexagonal ring form graphite. High pressure and temperatures contort these rings into a triangular pattern. Diamonds form because these patterns repeat. This process is called crystallization .

 

Almost all the diamonds used as gems come from the upper mantle.

Diamond embedded in kimberlite
Diamond embedded in kimberlite. Photo was taken at Field Museum of Natural History, Chicago, Illinois, USA, Credit:Wikimedia/James St. John

In subduction zones

Sometimes one tectonic plate slips under another. This event occurs in regions called subduction zones. When the plates slip past each other it presses the rocks. As a result, tiny diamonds form inside the rocks. Then, the rocks with tiny diamonds inside them come up to the surface. In contrast to how diamonds normally form, these form at a depth of only 50 miles (80 kilometers) below the Earth’s surface. The temperature here is just around 400°F (200°C). Clearly, the high pressure that builds between the plates plays a major role in forming diamonds. Interestingly, diamonds may also form during earthquakes due to the movement of tectonic plates.

 

These tiny and rough diamonds are not gem quality . In addition, diamonds formed in subduction zones are super rare. Sometimes, bluish diamonds form in the subduction zone. These blue stones have traces of oceanic crust in them.

A subduction zone can provide enough pressure to form diamonds
A subduction zone can provide enough pressure to form diamonds, Credit:Wikimedia/KDS4444

In asteroid impact sites

Earth has taken a few asteroid hits over the course of billions of years (the dinosaurs know). These asteroids pack huge heat and pressure during a collision. As a result, diamonds form. However, these diamonds look very different than the typical gemstones. They are tawny and grey. These diamonds, called lonsdaleite, are touted as the strongest natural substance ever. Meteor Crater in Arizona is a source of lonsdaleite.

 

Making jewelry with lonsdaleite is a no-go. First, it is not gem quality. Second, it is super rare. Thirty grams of lonsdaleite cost over 100 million dollars.

A piece of Diablo Canyon meteorite
A piece of Diablo Canyon meteorite at UCLA Meteorite Gallery. Lonsdaleite was obtained from this meteor, Credit:Wikimedia/Vahe Martirosyan

How do diamonds reach the surface of the Earth?

Rocks in the upper mantle are molten, and this mix of molten rock is called magma. It contains carbon-rich rocks called kimberlites. The carbon in the molten rocks becomes diamonds under high pressure and temperature. When the magma expands below the surface of the Earth, it erupts through vents and fissures in the crust. Later, the magma cools down inside these vents. As a result, pipelike structures of kimberlites form. Naturally, these structures are known as kimberlite pipes. These structures got their name because they were first found in the town of Kimberley in South Africa.

 

Finding a diamond in these pipes involves quite some luck. Only 1 in 200 kimberlites contains a diamond. Even then, the diamond might not be of good enough quality for jewelry.

 

What do we do with poor-quality diamonds? Toss them in the trash can? Not so fast. Diamond is the strongest natural substance. Hence, diamonds work like magic in grinders. Moreover, diamond drill bits and saws are favorites in the automobile industry. Additionally, nanodiamonds function as indicators in cancer drugs.

Do diamonds form from coals?

Coal is carbon. Diamond is carbon. Hence, coal can become diamonds? Yet, the theory has a major flaw. As noted earlier, diamonds were formed billions of years ago. Coal is the remains of plant life from tens of millions of years ago. Hence, diamonds are not only way older than coal, but they are also like great-grandparents relative to plant life itself.

How are lab-grown diamonds made?

First, mined diamonds take millions or billions of years to form. Second, they are hard to acquire. This makes mined diamonds rare and expensive. However, people want to own diamonds. Finally, in the 1950’s, scientists were able to “grow” diamonds in a lab. They developed a technique called HPHT (high pressure, high temperature). Scientists apply extreme heat and pressure to a tiny diamond (a slice of a diamond as thick as a human hair) and a carbon source (like graphite). This mimics the natural environment which favors the formation of diamonds. The carbon melts and crystallizes onto the diamond seed to form a lab-grown diamond.

 

Another method is called CVD (chemical vapor deposition). First, methane is introduced into a vacuum chamber . Then, microwaves break down the methane molecules into carbon. Eventually, the carbon atoms crystallize to form diamonds, in a process similar to how water vapor deposits on pieces of ice to form snowflakes.

 

Lab-grown diamonds are 100% authentic and gem quality. They grow in the same way as any natural diamond. They are much more affordable and sustainable. What’s more? Compared to the billions of years that a natural diamond takes to form, lab-grown diamonds form within weeks!

Lab-grown diamonds
Lab-grown diamonds are as authentic as real diamonds, minus the mining troubles, Credit:Wikimedia/Igor Stratichuk

In which countries are diamonds mined?

In conclusion, like many pretty things, a diamond comes at the cost of rarity. Firstly, they take eons to form. Secondly, mining diamonds is hard and dangerous. Thirdly, not all diamonds are gem quality. Yet, many countries make a fortune out of diamond production. These are the countries where the demand for diamonds is less. African countries like South Africa, Botswana, and Angola are hubs of diamond production. However, diamond reserves were found in Russia in 1957 (the same year as the launch of Sputnik). Currently, Russia produces the most diamonds. They have held their top spot for over a decade now. The surprise runner-up is Canada. Decades ago, geologists theorized that there would be diamonds to the north of Yellowknife, a Canadian city in the Northwest Territories. Explorers failed to find them, until 1991. Geologists Chuck Fikpe and Stewart Blusson found evidence of diamonds in that region. By the turn of the millennium, Canada was a major producer of diamonds.

Jericho diamond mine in Nunavut
Jericho diamond mine in Nunavut, Canada, Credit:Wikimedia/Tom Churchill

The USA buys 40% of all mined diamonds. Other major consumers include India and China, and some European countries.

Glossary

Upper mantle: Layer of molten rocks below the crust of the Earth. The crust floats on the upper mantle.

 

Crystallization: The process by which atoms or molecules bond to form repeating patterns or units to form a crystal.

 

Tectonic plate: The Earth’s crust is not a single mass. It is broken up into pieces, and these pieces are called tectonic plates.

 

Asteroid: Enormous rocks that move around in space; these can collide with planets to cause widespread geological changes.

 

Nanodiamonds: Very tiny diamonds, even smaller than a speck of dust.

 

Vacuum Chamber: A chamber from which all air has been pumped out.

 

Microwave: A wave of energy that can break molecules.

 

Gem quality: A set of parameters that are used to decide if a diamond can be used as gem or not.

Contributors

  • Anubhav Ghosh
    : Author
    I am pursuing my bachelor's in microbiology from Scottish Church College, Kolkata and the lab at my college is as close as my home is to me. My interest lies in molecular biology and cell signalling, and I want to be a professor when I grow up. I believe that what we see around has a fantastic science story in it. In my free time, I love to watch soccer. Writing for Smore Science gives me the chance to explore my take on explaining the science around me in ways that everyone can grasp.

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