Diamonds are often mistakenly referred to as rocks, but they are technically minerals. A mineral is a naturally occurring, inorganic solid that possesses a definite chemical composition and an ordered internal atomic structure. Rocks, in contrast, are typically aggregates of one or more minerals, or even mineraloids, without a fixed chemical makeup or specific crystalline structure.
The Elemental Makeup of a Diamond
A diamond’s unique properties stem from its fundamental composition. Diamonds are composed solely of carbon atoms. These carbon atoms are arranged in a highly organized, repeating crystalline structure.
Each carbon atom in a diamond is covalently bonded to four other carbon atoms in a tetrahedral arrangement. This strong, three-dimensional network of bonds is responsible for diamond’s extreme hardness, the highest of any known natural material. The atomic structure also contributes to its high density and exceptional thermal conductivity.
Genesis in the Earth’s Mantle
The formation of diamonds requires specific extreme conditions found deep within the Earth. Diamonds crystallize within the Earth’s mantle, at depths of 90 to 120 miles (140 to 190 kilometers) below the surface. This region is known as the “diamond stability field.”
Within these depths, carbon is subjected to immense pressures, around 45 to 60 kilobars (650,000 to 870,000 pounds per square inch). Temperatures in this environment are also extremely high, ranging between 1,650 and 2,370 degrees Fahrenheit (900 and 1,300 degrees Celsius). These conditions are essential for carbon atoms to bond together in the dense, crystalline structure characteristic of diamonds.
Bringing Diamonds to the Surface
After their formation in the mantle, diamonds reach the Earth’s surface through rare geological processes. This transport occurs primarily via a unique type of volcanic eruption, distinct from typical surface volcanism and originating deep within the mantle.
Diamonds are carried upwards within specialized igneous rocks known as kimberlite and lamproite. These rocks form in carrot-shaped geological structures called pipes or dikes, which extend from the mantle through the Earth’s crust. As magma from the mantle rapidly ascends through these pipes, it carries fragments of the surrounding mantle rock, including any diamonds present, towards the surface.
While diamonds themselves are minerals, kimberlite and lamproite pipes are the primary host rocks where diamonds are found. The rapid ascent of this magma is important, as it transports diamonds quickly enough to prevent them from reverting to graphite, their more stable form at lower pressures. Geologists focus on locating these specific types of volcanic pipes when searching for diamonds.