The diamond is highly prized for its extreme hardness and brilliance. This material is often mistakenly referred to as a “rock,” a “gemstone,” or a “crystal.” To truly understand the diamond, it is necessary to clarify its geological classification and the extreme conditions required for its formation. The journey of a diamond from deep underground involves a specific type of volcanic event that delivers the valuable material to the crust where it can be found.
Minerals Versus Rocks
A diamond is correctly classified as a mineral, not a rock, based on specific criteria used by geologists. A mineral is defined as a naturally occurring, inorganic solid with a definite chemical composition and a characteristic, ordered atomic structure. A diamond’s composition is pure carbon, with atoms arranged in a rigid, repeating tetrahedral lattice.
In contrast, a rock is fundamentally an aggregate of one or more minerals, sometimes containing non-mineral matter. Granite, for example, is a common rock composed of quartz, feldspar, and mica. Since a diamond consists of a single element in a consistent crystal structure, it is considered one of the building blocks of rocks.
Conditions for Diamond Creation
Diamond existence depends on a unique combination of high pressure and high temperature found deep within the planet. Diamond formation typically occurs in the Earth’s mantle, specifically within the mantle “keels” that anchor the ancient, stable continental crust known as cratons. This diamond stability zone exists at depths ranging from 140 to 250 kilometers (90 to 155 miles) below the surface.
The necessary temperatures for this transformation range between 900 and 1,300 degrees Celsius. Simultaneously, the immense pressure must reach 4.5 to 6 gigapascals (GPa), which is equivalent to 45,000 to 60,000 times the atmospheric pressure at sea level. These conditions provide the energy and compressive force required to transform the source carbon material into its crystalline form.
The raw material is carbon, which, at the Earth’s surface, is more stable as graphite. Under the mantle’s crushing pressure, the loose structure of graphite collapses. This process rearranges the carbon atoms into the dense, three-dimensional lattice that defines the diamond’s hardness.
Most natural diamonds are ancient, having formed over a period of 1 to 3.5 billion years within these stable mantle regions. They remain dormant until a rare and violent geological event transports them toward the surface.
The Volcanic Host Rock
Diamonds stored in the deep mantle are brought to the Earth’s crust by specialized volcanic eruptions that form a volcanic pipe or diatreme. The most significant igneous rock containing diamonds is called kimberlite, named after Kimberley, South Africa, where it was first identified as a diamond source. Kimberlite is an ultramafic rock, rich in magnesium and iron, and it originates deep within the mantle.
A less common but similarly important diamond-bearing rock is lamproite, which shares a deep mantle origin but has a different mineral composition. Both kimberlite and lamproite magmas ascend extremely quickly in an explosive, high-velocity event. This rapid ascent is necessary for preservation, as it bypasses the depth where diamonds would revert back to graphite.
The eruptions create vertical, carrot-shaped structures, known as kimberlite or lamproite pipes, that cut through overlying rock layers. These pipes act as conduits for the deep-source magma and are the primary source of mined diamonds. The diamonds are embedded within the rock matrix and must be mined and processed to liberate the crystals.
Over geological time, the primary kimberlite and lamproite host rocks can erode, releasing the durable diamond crystals. These diamonds are then carried away and deposited by rivers, streams, and glaciers, forming secondary or alluvial deposits. The original host rock remains the definitive source that delivered the diamonds from the deep Earth.