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.”