Today’s Factismal: The Wittelsback-Graff Diamond was placed on display at the Smithsonian in 2010.
Diamonds are fascinating, fabulous things. Their molecular structure gives them great strength and great clarity. And their imperfections not only make them sparkle, but also give them color. Add a little nickel and the normally clear, “white” diamond turns brown. Add extra pressure so that the atoms are closer together and it turns pink. Add a pinch of nitrogen, and you’ve got a yellow diamond. And add some boron, and you’ll get a blue diamond. So diamonds can tell us a lot about the place where they formed.
Though we can make small diamonds today by using intense pressure and heat or by building them up a layer at a time from a carbon mist, most diamonds come from nature. They typically form in the mantle, more than one hundred miles below us. Though this may seem deep to us, it is actually only about 2.5% of the way to the Earth’s center. To put that into perspective, if the Earth were an orange, diamonds would be made just under the rind (and our deepest wells would reach no further than zest).
As minerals like limestone and dolomite are carried deep into the mantle, the impurities (oxygen, nitrogen, and other elements) react with the surrounding mantle material, leaving behind carbon. Under the pressure of the mantle, the carbon atoms link into the three-dimensional crystal shape of diamonds. If the carbon links up in a shallower part of the mantle, then it forms sheets of graphite. And if it goes too deep, then the temperature is too high and the diamonds cannot form at all. And the change from flecks of carbon-rich detritus to diamonds doesn’t happen quickly. It can take anywhere from a billion to three billion years to form a diamond, depending on the amount of carbon available and the temperature of the surrounding mantle. As a result, diamonds form best under old, cold continental crust, like that under Africa and North America.
But forming the diamond is only half the battle; they have to make their way from the mantle all the way to the surface. This usually happens via a volcanic eruption. As those impurities that didn’t make it into the diamonds mix with the surrounding mantle material, they form new minerals, some of which are molten. The new, molten material is less dense than the surrounding mantle. And once enough of it accumulates, it is squeezed to the surface much like toothpaste being pushed out a tube. And, as the new magma erupts, it carries with it a few of those diamonds.
Eruptions like this have been found in Russia, in South Africa, in Australia, and even in North America. As a matter of fact, the world’s only diamond mine that is open to the public is in Arkansas. Known as the Crater of Diamonds State Park, ordinary people like you can go and dig for diamonds!
If you’d like to get involved with geologic citizen science and don’t want to spend the day digging for diamonds, then why not take part in the Geo-WIki project: