Superman lied to us. Over the years countless Superman comic books, TV shows and movies have shown the fabled Kryptonian crushing clumps of coal between the palms of his hand to turn them into shiny, sparkling diamonds. It makes for a great plot point, but here's the truth: it would never work.
It's easy to see where the idea came from, though. Diamonds and coal are both, at their base, different forms of the element carbon (C on the periodic table). And yes, pressure is a key part of what turns decaying carbon-based life forms such as plants into coal, as well as what turns carbon into diamonds. But the reality is just a little bit more complex than Superman's super-strength.
First of all, let's look at the chemical compositions of these two forms of carbon. Diamonds are essentially pure carbon formed into a crystalline structure. The rarer, colored diamonds do contain minor impurities (boron, for example, makes diamonds blue, while nitrogen turns them yellow), but those impurities exist on a scale of just one atom in a million.
Coal is also mostly carbon, but it is hardly pure. Coal also includes many other substances, including hydrogen, nitrogen, oxygen, sulfur, arsenic, selenium and mercury. Depending on the type of coal and its source, it will also contain various levels of organic materials — coal originates from decaying plants, fungi and even bacteria — as well as moisture. These impurities alone prevent coal from being turned into diamonds. (The impurities are also why burning coal produces greenhouse gases and contributes to acid rain and other environmental problems and why coal mining is so environmentally destructive.)
Beyond that, carbon requires a lot more than pressure to become a diamond. It also requires enormous amounts of heat. In fact, diamonds require a combination of heat (thousands of degrees) and pressure (130,000 atmospheres) that can typically only be found about 90 to 100 miles below the surface of the Earth, deep within the mantle. This heat and pressure work together to allow the carbon to form into the crystalline lattice structure that we know so well. When presented with this heat and pressure, each carbon atom bonds with four other atoms in what is known as a tetrahedral unit. This strong molecular bond provides diamonds with not just their structure but also their classic hardness. That bond would not be possible if impurities were present on anything but a superficial level.
If diamonds form so far below the surface of the earth, how do they end up on our fingers? The process started millions if not hundreds of millions of years ago when volcanic eruptions brought the diamonds closer the surface. Erosion, geological shifts, streams and other processes then scattered them further from their original eruption sites.
A few diamonds come from slightly different sources. Deep-sea oceanic tectonics has been linked to the creation of some particularly small diamonds. Asteroid strikes may have created some others, as millimeter-sized diamonds have been found in some craters. Both of these processes probably involved limestone, marble or dolomite rather than coal, according to Hobart King at Geology.com.
Diamonds aren't an Earth-bound phenomenon, by the way. King also points out that some nano-scale diamonds have been found inside of meteorites. But there's no coal in outer space, so once again these tiny diamonds were probably formed by pure carbon.
So no, it turns out that coal can't be turned into diamonds. Maybe that's why Santa leaves lumps of coal for bad little boys and girls. Unless Santa doesn't exist either? Nah, that's one legend that has to be true, right?
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