Some people think that Earth diamonds take billions of years to form. But the fact is that today we know that the process of forming diamond itself can just take hours and days under the kinds of pressures available deep inside the Earth. (Of course, then diamonds can lie in the ground for any length of time.)
Deep inside the Earth, there exist pressures of 5GPa and up to 350 GPa and higher. Under those conditions, we now know diamond can crystallize within days or even hours. Diamond as we know it — solid diamond at reachable depths — then results when it is cooled very fast. Because if it is cooled too slowly, graphitic, non-diamond rock results. (So only if an eruption helps shoot up material to the Earth’s cooler layers very fast — no slower than within a few hours — then the unique crystal called diamond is locked in. If it takes longer than a few hours, no good diamond results.)
At Diamond Foundry, we are re-creating conditions under which diamond forms. It turns out technologies for growing diamond today can manage to approximately replicate the speed of Earth’s formation process — but cannot yet beat it. Earth can sustain much higher pressures at deeper depths than any man-made technology is capable of today. Plus we have not yet found a way of growing diamond at lower pressures in ways that are distinctly faster than higher-pressure technologies are capable of.
Read more here at Smithsonian: “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.”