Where Do Diamonds Come From ?

Experiments and the high density of diamonds tell us that they crystallize at very high pressures. In nature this means that diamonds are created by geologic processes at great depth within Earth, generally more than 150 kilometers down, in a region beneath the crust known as the mantle. Other processes, explored later in this exhibition, bring diamonds to where people can find them.

This diagram shows the interior structure of Earth. The three concentric layers -- the core, mantle, and crust -- formed within a few hundred million years of Earth's coalescence 4.5 billion years ago. The core is primarily an iron-nickel alloy and makes up a large fraction of the mass of Earth. The vast mantle is sandwiched between the core and the thin crust and is composed predominantly of magnesium and iron silicate minerals. Our planet's crust is a thin, rocky skin. Diamonds can form in most of Earth's interior but not near its surface, where graphite is the stable form of carbon. Indeed, diamonds only survive at Earth's surface because great heat is required to break down the diamond structure.

The upper mantle is slightly plastic, which allows it to circulate slowly in a creeping, convective flow that helps drive the surface motion of Earth known as "plate tectonics."

The cross section shown here provides a closer look at Earth's crust and underlying mantle. The crust can be divided into ocean basins, underlain by a thin layer of dense, basaltic rock, and continents, formed of a much thicker but less-dense layer of granitic rocks. Just below the crust is the portion of the mantle called the lithosphere, which is rigid and acts like rock. Below this is the asthenosphere, a more plastic, flowing region that enables the overlying crustal plates to move in what is known as plate tectonics.

The plot of pressure and temperature shows the conditions at which either diamond or graphite exist. The general conditions present in the Earth are described by curved lines called geotherms. Note that there are two geotherms: Because the continental crust is old and thick, conditions are somewhat colder in and beneath it than beneath the much younger ocean basins. Diamonds can form at depths as shallow as 150 kilometers beneath the continental crust, while beneath oceans they need depths of at least 200 kilometers, as shown by the diamond boundary on the cross-section.