What makes diamonds so hard?

The chemistry behind one of the world's strongest substances.....
20 September 2022

DIAMOND RING

A solitaire diamond ring

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Question

We know that diamonds are made up solely of carbon, but so are a lot of other things. What is it about the arrangement of carbon in diamond that makes it so strong, compared to something like graphite?

Answer

Kathryn Harkup answered this question...

Kathryn - Diamonds are very special. It is a unique combination of carbon atoms, in that every carbon atom within a diamond is connected to four others. And those four, are connected to four others. So a single diamond on a ring is in theory, one molecule of carbon, which is quite an extraordinary thought. You don't normally expect to see a molecule of something, but that's the kind of scale it's operating on. And all of this interconnectedness means that it is very difficult to chip off a bit of carbon, which gives it its strength. This arrangement also gives it its beautiful clarity and its lustre and reflects light. A lot of what we attribute and what we love about diamonds, is their visual impact and their cultural associations rather than their physical properties.

Chris - Where do they come from?

Kathryn - They come from very, very deep within the Earth. So they were formed many millions of years ago. And the only reason they ever make it to the surface was if many, many hundreds of thousands of years ago, a volcano happened to push through this particular bed of rocks. So, if the temperature wasn't too high the pressure wasn't too crazy and in flux, as it shifted right up to the surface of the earth that it destroyed the diamonds, then you might find diamond deposits on the surface of the earth. So they come from deep, deep, within the earth surface. They're very, very old, but of course you could just make some in the lab these days.

Chris - One thing I've always wondered. And, this sounds like a bit of a daft question, but it's a genuine, straight-up question. If diamonds are the hardest thing, what do you cut them with?

Kathryn - Other diamonds. So it's a real skill, cutting diamonds. The people that do it, they are artists, they train for a long time. And so you find that, because the crystals that you dig out the ground are not perfect. These are natural substances. They all have flaws. They will have cracks in them that you can take advantage of, but also you want to avoid in the finished product. So spotting where all of those flaws are that you can use and manipulate and shape and cut into another diamond, using a diamond, it's all very difficult and lots of planning goes into it. Because huge amounts of money can be lost if you make the wrong cut.

Chris - I can imagine. That's fascinating. Thank you for that. Peter Haynes?

Peter Haynes - Well, I'm interested in this idea of diamonds, right? So do you get anything like diamonds but made of other elements? You've said that a diamond is like a single large molecule of carbon. Do you find other single large molecules of other elements?

Kathryn - You can get single crystals of metals that are highly, highly, organised and grown in specific ways. They use it a lot for engineering purposes, because in theory there are no flaws within this metal. So there's no source of cracks and they're very hard wearing.

Chris - Are they called metallic glasses? I did meet a material scientist who told me that they're great for golf clubs.

Kathryn Haynes - Wow. They must be expensive. I've heard them being used in like jet engines for the blades of a jet, because obviously they're under an awful lot of pressure and they have a lot thrown at them. So yes, they won't look as beautiful and clear as diamonds simply because they're metals and the electronic structure within is very different. But yes, in theory, you can have not specific diamond structures, but highly organised, massive single molecules of other elements.

Peter - ​Okay, thank you.

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