This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Humans have been captivated by diamonds, the planet’s hardest natural material, for nearly five thousand years. In Egypt, they were incorporated into the ankh, symbolizing the sun. People in ancient India thought they must be created by lightning, and attracted it; they also expected the stones could stave off danger (a belief somewhat at odds with the lightning-attractor thing). Greeks and Romans found the stones much more sacred, seeing them as tears of the gods. Rather poetically, they also thought the gems might be shards of fallen stars. At various times, people have thought diamonds to be capable of conferring invincibility, of healing, and of sealing the deal on romantic love.
The truth of diamonds is almost as extraordinary as all of those beliefs. They are formed of the stuff of life itself: carbon. Extraordinarily hard, they can withstand enough pressure to recreate the extreme conditions under which they were born; and yet, subjected to the right combination of heat and oxygen, they’ll vanish in a puff of carbon dioxide. They form naturally in only a few places on Earth: deep beneath continental cratons, or in the shock of a meteorite strike. Most of them are billions of years old, and we’re not even sure if they’re still being formed beneath the crust today. And they’re brought to the Earth’s surface by some of the most bizarre eruptions in our planet’s history.
No ordinary volcano would do. Most of them are too shallowly-rooted to get at the deep places, roughly 150 kilometers beneath the crust, under the keel of continents, where diamonds form. And even if they do, their magma is too hot, too full of oxygen, too slow to bring a diamond successfully to the surface. You need a magma that’s ultramafic, far more basic than basalt. It must be somewhat cool (for deeply-buried, iron and magnesium-rich magma), no more than around 1,300°C. It has to contain a high percentage of volatiles like water or carbon dioxide, in order to propel the stuff to the surface. And it needs to move like no eruptions humans have ever witnessed, in order to sweep diamonds up from the great depths where they originate up to the surface.
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There are only three types of magma on the planet that can manage it, at least that we’ve identified so far: kimberlite, lamproite, and lamprophyre. All of them are ultrabasic. And most of them erupt in very odd ways. Kimberlite and lamproite especially like to power through the old, cold crust in and near cratons, expending vast energy in dashing to the surface and then off-loading their cargo in short-lived and small but very vigorous eruptions. Erik Klemetti has done a fabulous job describing how kimberlite volcanoes erupt. I couldn’t find a similar resource for lamproites, so I’m reading up on them and will have a post on them shortly.
Most of the volcanoes that contain diamond-bearing rocks are old, but the diamonds that hitched a ride with them are far older. The vast majority are between one and three billion years old. So if someone gives you a natural diamond, as you admire its sparkle, marvel at the vast swath of time this little stone has witnessed.
They may not be tears of the gods (although given the right conditions, they will evaporate like tears) or shards of stars (although some stars may be made of them), but they are remarkable little stones nonetheless. They have a rich and complex history, plus eventful travels, that more than repay a day’s reading. Please check out the links below.
And for those who aren’t wealthy enough to afford a diamond of their own: keep in mind that cubic zirconia is just as sparkly and will be happy to stand in.
References and Further Reading:
Geology of Diamonds
Diamond! (This is a marvelous article that covers all the bases)
Recent Advances in Understanding the Geology of Diamonds (More technical, but still marvelous)
The Crazy Eruptions That Spit Up Diamonds (Best post on kimberlite eruptions ever)
Mythology of Diamonds
Lord Shiva and the Hope Diamond
Human History of Diamonds