Hot and Heavy – The Truth about Diamonds

Humans are brilliant animals. Over the eons, we have used our ingenuity and problem solving to craft a civilization that, as imperfect as it can be, does a pretty effective job of keeping us out of the food chain. However, clever as we may be, we have a few weaknesses. Chief among them are greed, the ability to be manipulated, and an inexplicable fondness for shiny things. The perfect symbol of these primeval flaws is the modern diamond engagement ring. 

Now, don’t get me wrong, scientifically speaking, diamonds are impressive things. They are made from the same single ingredient as coal: carbon; but we proudly put one in jewelry, while we are content to throw the other in the fire.

A common misconception about diamonds is that they form from coal as the Earth does its thing by applying heat and pressure. In fact, if coal has ever had anything to do with diamonds, it has been incidental and insignificant, and has produced some pretty lousy diamonds. If you need proof, consider that most natural diamonds on Earth are between 1 and 4 billion years old, while coal (which formed from dead plants before bacteria evolved to have the ability to digest wood) has only been around for about 500 million years.

So if coal isn’t the culprit, where do diamonds come from? It turns out there are several answers:

Deep Source Eruptions – Most diamonds form in the layer of semi-molten rock that the Earth’s crust floats around on. We call this the mantle, and it is not a place you want to find yourself in. The section of the mantle where diamonds generally form is around 150 km (90 miles) below the surface, so for starters it would be hard to get out of. Second, it is hot, around 1,050 C (2,000 F). To make matters worse, the pressure is about 725,000 pounds per square inch. 

While this would really, really suck for any human without a magical ship from a crappy sci-fi movie, it is quite a swell place for diamonds to form. Carbon dioxide, trapped in the mantle when the Earth first formed, undergoes a “redox reaction” due to the extreme conditions. The carbon oxidizes (rusts, in its own weird way) and gains electrons. When the pressure is suddenly reduced very rapidly – say, when a weak section of Earth’s crust drifts over top of it – the molten rock erupts to the surface, the carbon condenses, and forms diamonds. This process is responsible for all the commercial diamonds in the world.

Subduction – Diamonds can also form as the Earth’s tectonic plates bump and grind. As dense oceanic plates grind underneath lighter continental plates, the resulting heat and pressure can produce diamonds. This process could involve coal as a carbon course, but it is more likely that the material comes from limestone and other rocks.

Impact Sites – Being underneath an asteroid as it impacts the Earth is one of the few places you could find yourself that would be worse than the mantle, but, if you can run away in time and get back before anyone else, you could score big. The heat and pressure from the impact has the ability to metamorphosize carbon and produce diamonds. The catch is that the biggest ones you are likely to find, thanks to the keen smashing ability of objects from space, would be in the 1 mm range. That would make for a pretty insulting engagement ring.

Space – Diamonds can also form in space as objects smash into each other at high speed. The process is basically the same as it is at impact sites on Earth. Except in space, no one can hear your fiancée shriek with delight… well, maybe David Bowie.

The curious thing about diamonds is that, as much as we value them, they aren’t at all rare. Since 1870, when massive diamond deposits were discovered in Kimberley, South Africa, the world has basically had all the diamonds we could want. In fact, last week a team of chemists at John Hopkins University reported that the complex “redox reaction” that forms diamonds could be produced by water under the right conditions. Given the amount of water on Earth, there is probably a shocking amount of diamond mines just waiting to be discovered.

The only reason diamonds are expensive is because of that pesky manipulation and greed we started the article with. After the diamond rush of 1870, a company that came to be called DeBeers bought up all the diamond producing mines in existence and began to sharply limit the supply to keep prices high. They coupled that with a 20^th century ad campaign that tried to convince us all that the only way to show someone you love them enough to marry them is to put a shiny hunk of carbon on their finger.

Dropping 3 month’s salary (cue the hysterical laughing) on a ring seems a lot less special when you realize there is basically an infinite supply of replicates in some rich guy’s vault.