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Why are diamonds clear, but coal black?

Maggie Koerth-Baker answers a new Science Question from a Toddler.

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Russia reveals large deposit of "extra-hard" diamonds in asteroid crater

The Russian news agency ITAR-TASS reports that the government has declassified a large deposit of diamonds, located in a meteorite crater formed 35 million years ago. The unique composition of these "extraterrestrial gemstones" could make them uniquely valuable for the technology industry:

According to Academician Pokhilenko, "the value of impact diamonds is added by their unusual abrasive features and large grain size." "This expands significantly the scope of their industrial use and makes them more valuable for industrial purposes / in metalworking, in production of efficient semiconductors, etc./," he said. In addition, as yet, impact diamonds with similar specifications have not been discovered anywhere else in the world. Thus, experts speak about their extraterrestrial origin and claim that Russia becomes a monopoly owner of unlimited supplies of this unique raw material, which is of highly demand in advanced technologies. Scientists forecast, this raw material reserves "would be enough for the entire world for 3.000 years." Use of these minerals in the manufacturing industry is capable of a technical revolution.
The diamonds are described as "extra-hard." #thatswhatshesaid

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 Geology.com'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