How is a diamond formed? The millennia-long journey of the ultimate precious gem

Deep within the Earth’s mantle, more than 150 kilometers below the surface, and under extreme conditions of pressure (≥4 GPa) and temperature (950–1400 °C), one of nature’s most fascinating materials is formed: the diamond. Most natural diamonds are over a billion years old. Their formation takes place in ancient cratonic regions, where pure carbon can crystallize into a cubic structure thanks to stable and favorable geothermic conditions.

Diamonds form in a metastable equilibrium within the upper mantle, embedded in peridotitic or eclogitic rocks. Their growth is a slow process, influenced by the availability of carbon-rich fluids and the local geothermal gradient. These precious stones remain encapsulated in their host rocks for geological eons, until deep magmatic events carry them toward the surface.

The ascent occurs through rare and violent volcanic eruptions that create kimberlite or lamproite pipes. These rapid and explosive events are essential: only a fast ascent preserves the diamond’s stable structure, preventing it from reverting to graphite. The resulting formations—known as kimberlite pipes—are the world’s primary sources of mined diamonds.

In a small percentage of cases, the crystallization process involves variations in chemical composition or lattice distortions, giving rise to fancy color diamonds. Various factors influence coloration: aggregated nitrogen causes yellow or orange hues; boron produces blue; structural-plastic deformations during growth or ascent result in pink or red tones. Green shades typically stem from exposure to natural radiation in the surrounding host rocks.

Each diamond is thus an extraordinary product of a unique geological process—a rare balance of deep time, pressure, temperature, and tectonic dynamics. A silent journey that begins in the depths of the Earth and concludes only when the gem emerges into the light—intact and unchanged—a crystallized witness to the evolution of our planet.