Why are diamonds clear while coal is black?

In the Superman folklore, he turns a lump of coal into a diamond by compressing it in his fist, cementing the idea in popular culture that diamonds are compressed coal. But in reality, while both diamonds and coal are both made of carbon, they are formed differently and at different locations underground. Coal and other fossil fuels are formed in the Earth’s crust (see the comments to an earlier post for some excellent information from readers about how fossil fuels form) but diamonds are created at much greater depths within the Earth’s mantle.

Experiments and the high density of diamonds tell us that they crystallize at very high pressures. In nature this means that diamonds are created by geologic processes at great depth within Earth, generally more than 150 kilometers down, in a region beneath the crust known as the mantle.

Diamonds can form in most of Earth’s interior but not near its surface, where graphite is the stable form of carbon. Indeed, diamonds only survive at Earth’s surface because great heat is required to break down the diamond structure.

But if both are made of carbon, how come that diamonds are bright and shiny while coal is black? Maggie Koerth-Baker provides an answer.

Coal is the compressed remains of ancient plants, dinosaur swamps sitting in the palm of your hand. But there are diamonds that are older than terrestrial plants. That fact alone should tell you that diamonds are not actually made from compressed coal. Instead, diamonds are probably formed deep in the Earth—much further down than the levels at which we find coal—where heat and pressure fuse atoms of carbon together into crystalline structures. Later, those crystals get vomited up from the depths with the help of volcanic vents.

While both diamonds and coal are made from carbon, coal also contains a vast number of impurities such as sulfur and mercury that are released into the atmosphere upon burning, which is why it is not considered a source of clean energy. The so-called ‘clean coal’ technology seeks to find ways to remove some of the pollutants before burning and reduce the emissions of things like sulphur dioxide (a major source of acid rain) and carbon dioxide after burning.

But the real difference lies in the different ways that the carbon atoms are bound together in diamonds and coal and how that affects the way that light is absorbed and reflected by their surfaces. As Koerth-baker says, “The difference between sigma-bonded diamonds—which throw off photons outside the spectrum of visible light—and pi-bonded graphite—which absorbs all colors of visible light is extreme.”


  1. corwyn says

    So, why are diamonds clear, while coal is black? This didn’t even address the question.

  2. coragyps says

    Just convert all the coal- fired power plants to burn diamonds, and we wii be free from that mercury/lead/sulfur pollution!

  3. coragyps says

    Corwyn – it’s all in that sigma-bond versus pi-bond clause up above. Visible- light photons will interact with pi electrons, but don’t have the oomph to bump sigma- bond electrons out of their place.

    Yes, “oomph” is a real chemistry word. Ernest Schroedinger said it all the time.

  4. khms says

    There’s a reason diamond mines are usually former volcanoes. (And coal mines aren’t.)

    However, it’s not as if diamonds are free from pollutants. That’s where colored diamonds come from, for example.

    And yes, putting coal under enough pressure is likely to produce diamonds, though, as the process for industry diamonds shows, there are easier ways to do it.

  5. dogfightwithdogma says

    I enjoyed the Superman movie, but to this day I still curse it for the misconception it perpetuated. I encountered this misconception – that diamonds form from the compression of coal – countless times during the 21 years I taught high school earth science. I hope that I succeeded in eliminating this misconception from the minds of my students, but given the staying power of misconceptions I can’t be certain I met with success in every instance.

  6. says

    Just as interesting, how did Buckminster Fuller and the inventor of the traditional soccer ball come up with the “buckyball” shape, now called a fullerene, without knowing the third form of carbon, C60? It wasn’t discovered until 1985.


    Take a soccer ball made of 20 hexagons and 12 pentagons. At the sixty points where three faces meet, imagine a carbon atom, and on the lines between two adjacent faces there is an atomic bond. That’s a buckyball.

    C60 has been detected in the nebulae of supernovae, and can be found in the burning of carbon rods during arclighting. C60 can be extended into C70, 80, etc, by adding ten atoms at a time. The tubes created from this (called CNT graphene) are the small tubes being hypothesized as the basis for nanocomputing.


  7. ChristineRose says


    Pretty hard, actually.

    The consensus in the synthetic gem industry is that LifeGem is a fraud. All the growers have tried making gems from ashes, and none of them have been able to do it. There’s just too high a ratio of things that are not carbon. After all, ashes are what’s leftover after the carbon has been burnt off.

    There’s no identifiable biological material in a synthetic diamond. It’s at least possible that they mix a tiny bit of the dirty carbon from the cremains in with the purified carbon that all diamond growers use, but they can’t even prove that.

  8. permanentwiltingpoint says

    Here in Germany at least, it has become a quite common service in funeral parlours to press you a diamond from the ashes of your beloved.

  9. Peter B says

    In his penultimate paragraph Dr. Singham appears to say, “The so-called ‘clean coal’ technology seeks to find ways to remove some of the pollutants before burning and reduce the emissions of things like … carbon dioxide after burning.”

    With cleaning at 100% coal is pure carbon. With complete combustion only carbon dioxide is formed. I.e. “clean coal” can not reduce carbon dioxide production. Careful burning can reduce carbon monoxide and various oxides of nitrogen.

    And then it get complicated. Higher temperatures are more efficient when combustion is used to spin turbines. But higher temperatures start breaking nitrogen bonds. Result: thermodynamic efficiency and smog. Catalytic converter? The coal had better be *very* clean.

  10. Mano Singham says

    Peter B,

    While I agree that carbon dioxide must necessarily be produced when burning coal, I thought that part of clean coal technology is not just the cleaning of coal but also involved the capture of the produced carbon dioxide so that it is not released into the air.

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