Diamonds do not come from coal

Okay, maybe I'm an idiot, but this is one of those facts I'd missed until recently. Despite the impression you may have gotten from grade school and/or old Superman cartoons, diamonds are probably not lumps of coal that just got compressed real good—at least, not in exactly the way you might imagine.

Diamonds are made out of carbon, but the best evidence suggests that they form far more deeply down in the Earth than coal does. Instead of coal being smushed into diamonds, imagine something more like those "grow crystals out of Borax and water" experiments you did in grade school. Only, in this case, the experiment is performed in the fiery depths of Hell, as very un-coal-like atoms of carbon are compressed and heated deep in the Earth's mantle until they start to bond together and grow into a crystalline structure.

Once the crystals are formed, they get to the surface of the Earth via volcanic eruptions.

The really interesting thing about all of this is that it's one of those ideas that's very hard to verify. Diamonds form at a depth we can't go observe directly. All we have to work with is indirect evidence. Because of that, nobody knows exactly where the necessary carbon to make diamonds comes from. This is why the "diamonds are coal" story exists. Some scientists think the carbon is stuff that's existed in the Earth since this planet was formed. Others think it might be coming from terrestrial carbon that got shifted down to the lower levels via plate subduction—although, even then, we're talking about carbon, but not necessarily coal. It could be a combination of both. Either way, the mental image of smushed coal doesn't quite work.

Read the American Museum of Natural History's explanation of where diamonds come from

Read an interview about diamonds with the curator of the U.S. National Gem and Mineral Collection

Thanks to a story written by's Hobart King for busting the myth and inspiring to me to read a little more on this

Image: Diamonds, a Creative Commons Attribution (2.0) image from kimberlyeternal's photostream


    1.  Crushing coal :)

      Well, sort of.  Carbon is subjected to very high pressure and temperature in an anvil cell.  Confusingly, the best-known anvil cells are diamond anvil cells, but those don’t make diamonds, they’re made from it.

  1. Diamonds are the hardest natural material, so nope… nothing smushed about them. However, if struck at *certain* angles, they can crack. 

    @jonnygoldstein You are probably thinking of cubic zirconia re: “human created” diamonds. Those are made out of zirconium dioxide. There are other, newer ways to recreate diamonds artificially, too. 

    1. I’m pretty sure he was asking about the actual man-made diamonds, not the cubic zirconia type non-diamonds. I’m actually pretty interested myself in the state of the carbon they use for the process.

      Though, really, let’s be honest – diamonds are just inferior moissanite in every way except pure hardness, and moissanite is pretty competitive there too.

    2. Wurtzite boron nitride FTW!
      I think graphene is more durable than diamonds, albeit discovery of it’s strength is fairly recent.

  2. So what Ferris Bueller told us about Cameron and the piece of coal isn’t true? John Hughes lied to me? What else do I have to question now?

  3. Diamonds are made of highly-organized, super-tightly packed molecules of marketing.

    1. Amethysts were valued as highly as diamonds, emeralds, rubies and sapphires until somebody found an endless supply in Brazil.

  4. Some scientists think the carbon is stuff that’s existed in the Earth since this planet was formed.

    Careful there – you are nearing the theory of Abiogenic petroleum,  a crackpot theory favored by Creationists and Peak Oil deniers that the petroleum in the earth was formed at the earth’s creation and is slowly seeping up and will never run out.

    While coal comes from land plants, petroleum comes from ocean algae which has been around longer and may have been subducted.

    1.  Uh, I really don’t think that that’s where Maggie was going; even without believing in abiogenic petroleum, obviously there had to be carbon on earth before there was life, and it wasn’t all necessarily distributed on the earth’s surface.

      1. “Thomas Gold was a scientist, a long shot away from a crackpot.”

        Just because someone is recognized as a scientist doesn’t mean that they cannot be crackpots. Would you like examples of crazy scientists?

        Currently all advocates of “abiogenic petroleum” are crackpots or have a hidden agenda.

        Your wikipidia link reads as a puff piece that fail to mention how ridiculous many of his theories are and how they were derided at the time he proposed them, as well as barely mentioning his plagiarism issues.

        As one small example, consider his theory that coal is made by bacteria from the methane that flows underground, as opposed to being fossilized plants. This flies in the face of coal containing fossils of recognizable plants.

        1. I don’t have a hardened opinion or expertise with respect to origins of various hydrocarbons. I was just pointing out that Tommy Gold was a scientist to the bone. He held some unorthodox views; some held up, some didn’t. He was a scientist long enough to shift or let go when evidence contradicted those views, go out on a limb when there were interesting possibilities, and to do the work to support his views. 

          I know enough about his work on origins of hydrocarbons to recognize  that “…petroleum in the earth was formed at the earth’s creation and is slowly seeping up…” is a misrepresentation of his viewpoints and work. He would not deny that coal often contains fossils of recognizable plants. I’ve met him, read some of his work, used scientific facilities he helped develop and managed and have seen him speak. It’s a good bet he was more careful, rigorous, rational and reasonable (able to be reasoned with) than you, but I’m guessing he wouldn’t give you the time of day unless you had something to say he hadn’t heard or thought about before.

          It’s fine to call him crazy and a crackpot, MonkeyBoy. This brand of criticism was freely expressed when he was able to defend himself. But I’m not going to let it go by without mentioning that he was a scientist.

  5. I liked the part where Superman crushed the lump of coal and when the diamond came out it was already cut and polished perfectly!  

  6. Probably the diamonds-from-coal trope came from the fact that both are mined, and that you have the image of the precious mineral being created directly from the common fuel. Plus, as already mentioned, Superman creating one directly from the other has been a recurring theme in comics until relatively recently. 

  7. I feel like I should have something intelligent to add, but I got a B- in mineralogy and a C in petrology and have avoided the topic in subsequent courses and research. But I do remember a lot about kimberlite pipes, where most African diamonds come from, which are pretty cool as far as geology goes and the ones in Africa have unbelievably deep mines going into them (they are also often excavated as open-pit mines creating the deepest man-made holes in the ground in the world).

    Also, the professor who gave me a C (and who gave me a 6/100 on one of the exams) showed me some of the exceedingly rare rocks from his research in his office, including ones chock-full of (non-gem-quality) diamonds that formed in the mantle. There are very few ways for rocks to come to the surface intact from the mantle (without being metamorphasized) which is why there’s so little known about this stuff.

  8. So why isn’t Diablo surrounded by diamonds, Blizzard?  GOD I hate inaccuracies in my fantasies!

    1. I learned that this weekend at the jeweler! It’s a little worm that gets in, and the oyster reacts to the foreign protein.
      As the jeweler said, there’s a lot of sand around where oysters live.

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