Diamonds are formed deep within the Earth about 100 miles below the surface in the upper part of the mantle. The temperature in the upper mantle is extremely hot but varies. The temperature near the crust can be around 1832 degrees Fahrenheit (1000 degrees Celsius). Around the area where the lower mantle borders the outer core, the temperature is about 6692 degrees Fahrenheit (3700 degrees Celsius). There’s a lot of pressure, the weight of the overlying rock bearing down, so that combination of high temperatures and high pressure is that’s necessary to grow diamond crystals in the Earth. As far as we know, all diamonds that formed in the Earth Formed under those kinds of conditions and, or course, that’s a part of the Earth we can’t directly sample. We don’t have any way of drilling to that depth or any other way of traveling down to the upper mantle of the Earth.
So if diamonds form at depths we can’t reach, how do they appear at the surface? The diamonds that we see at the surface are the ones that are brought to the surface by a very deep-seated volcanic eruption. It’s a very special kind of eruption, thought to be quite violent, that occurred a long time ago in the Earth’s history. We haven’t seen such eruptions in recent times. They were probably at a time when the Earth was hotter, and that’s probably why those eruptions were more deeply rooted. These eruptions then carried the already-formed diamonds from the upper mantle to the surface of the Earth. When the eruption reached the surface it built up a mound of volcanic material that eventually cooled, and the diamonds are contained within that. These are the so-called Kimberlites that are typically the sources of many of the world's mined diamonds.
One of the things we know, therefore, about any diamonds that were brought to the surface is that the process of the Kimberlite eruption bringing the diamonds from the upper mantle to the surface of the Earth had to happen very quickly, because if they were traveling too long and too slowly they would have literally turned into graphite along the way. And so by moving quickly they essentially got locked into place into the diamond structure. Once the diamonds have been brought from high temperature to low temperature very quickly—and by quickly, we mean in a matter of hours—these eruptions, these Kimberlite pipes moving to the surface, may have been traveling at rates of 20 to 30 miles per hour. Once the diamonds are brought to the surface and cooled relatively quickly, those carbon atoms are locked into place and there’s just not enough energy to now start rearranging them into graphite.
Frequently asked questions include how long does this process take? We really don’t know how long it takes. There have been attempts to try and date inclusions in different parts of diamonds, and those attempts have been largely unsuccessful. It may be that diamonds form over periods as short a time as days, weeks, months to millions of years. Typically, as with many crystals that grow on the Earth, it’s not a continuous process. The diamonds may start to grow and then there may be an interruption for some reason — a change in conditions, temperature, pressure, source of carbon, etc—and they could sit for millions, hundreds of millions of years, and then start growing again. That's part of the problem of trying to put some sort of a growth period on them; things don’t always occur continuously in Earth.
