A diamond: A perennial, ubiquitous human symbol of matrimony and unabashed luxury. But this crystalline allotrope of carbon goes through quite a lengthy and intense journey from creation to ornamentation. Since we cannot directly study the Earth’s interior, scientists can only provide educated guesses about conditions that lead to a diamond’s formation. They estimate that diamonds form in Earth’s upper mantle, around 90 miles below the surface. This depth has the proper range of conditions conducive to diamond formation: A pressure approximately 362,000-942,000 pounds per square inch and a temperature between 750 and 3,200 degrees Fahrenheit. Carbon-containing minerals provide the raw materials and this unrelenting pressure and temperature squeeze the carbon atoms into a lattice crystal configuration—and a diamond is born. It is not known how long it takes for a diamond to naturally become an appreciable size, estimates range from days to millions of years. And diamond crystal growth isn’t necessarily a continuous process, it could progress over a few days, stop for millennia, then start up again. The diamonds that are found at and near Earth’s surface usually arrived billions of years ago carried up through a volcanic eruption. This eruption must be especially rapid—the material must cool within a matter of hours or it will lose its crystalline structure and become the carbon allotrope graphite. These cooled lava tubes, known as Kimberlite pipes, are found on every continent except Europe and Antarctica, though most of the diamonds that become jewelry come from Kimberlites located in South Africa, Australia, Botswana, Russia and the Congo Republic.