Really, you can interpret the question above in two ways. You can go with, ‘How does planet earth make diamonds?’ or, ‘How do people make diamonds?’ Since the principle is the same (even if the execution is different) I’ll explore both here. And we’ll start with the earth.
That seems only fair, don’t you think? After all, not only was she making them first, but if she hadn’t we wouldn’t even know about them! Even if we did – perhaps because we’d started making them in the lab – they wouldn’t hold the same significance. They wouldn’t grace wedding rings nor would we talk about them being a girl’s best friend.
How The Earth Makes Diamonds
There is only a very small slice of our planet in which most diamonds form, which runs from about 140 to 190 kilometers (or 87 to 118 miles). That’s only about half a percent of the actual earth’s radius! Why is this the goldilocks zone of diamond formation? Because both the pressure and the heat are just right for carbon minerals to transform into diamonds.
Note that I said ‘carbon minerals’ and not ‘coal’. That’s because despite what you’ve been told, coal rarely becomes diamonds. It’s not that coal couldn’t become diamonds. It could. Instead, it’s more that there isn’t that much coal down at the right depth. Coal seams generally only go about 2 kilometers and there isn’t enough pressure or heat down there.
Even with the intense pressure of having 140 kilometers of rock sitting on top, it still takes a while for diamonds to form. How long? Somewhere between one to three billion years. That makes them about ten times older than dinosaurs!
And their formation is only half the battle. Because they still have to make their way up to us. If you thought we could dig down that low, you overestimate our technological prowess. The deepest we’ve ever managed to dig is 12 kilometers (or 7 ½ miles). The Russians managed that at the Kola Superdeep borehole. That’s not even 10% of the way.
So how do diamonds make their way to the surface? Volcanoes. And not any ordinary volcano either. Those won’t do for this extraordinary rock. Their magma chambers only go about 50 kilometers down. The ones that go the necessary 150 kilometers are much rarer. That means only a small percentage of diamonds are brought up to the surface, which adds to their rarity.
How People Make Diamonds
Now, we can’t wait around a billion years for diamonds to form in the lab. I mean, I don’t know about you, but I’ve got dinner plans. Fortunately, scientists worked out ways that go a little bit faster than that. Modern diamonds can be made in as little as three weeks.
The two most used methods are known as ‘High Pressure High Temperature’ (HPHT) and ‘Chemical Vapor Deposition’ (CVD).
HPHT is how you’d imagine you’d build a diamond. In effect, you replicate what happens underground. You take a bunch of steel anvils, heat carbon to ridiculously high temperatures and then compress. Hey, presto! Diamonds.
The CVD method is a bit different. Here you don’t crush carbon together and force it into a diamond. Instead, you grow them. All you need is a special container, super-heated methane and hydrogen gas and some seed diamonds. The methane then converts to carbon, bounces around and occasionally strikes the seed diamonds.
Sometimes they’ll stick. If they align with the diamond’s underlying matrix, then they get lodged in place and become part of the diamond. If they don’t, then they’re not as firmly attached and the hydrogen will scour them off again. In this way, the diamond doesn’t end up with all sorts of impurities which would make it opaque, weak and brittle.
Did you know diamonds produced this way are purer than natural ones and will lack almost all faults of inclusions? In fact, that’s one of the ways you can often tell that a diamond is factory made!
So there you have it. That’s how diamonds are made by nature and humanity. It’s a fascinating process that takes some extreme conditions to give us these stones that we know and love. It also adds a nice bit of history to those rocks on our fingers or around our necks. Because that isn’t any stone. It’s one that has seen the inside of the earth, is older that multi-celled organisms, and has been part of a volcanic eruption. Now that’s impressive!