The public perceives a diamond as a brilliant, perfectly cut gemstone sparkling under a light. However, a diamond found beneath the Earth’s surface looks nothing like this polished final product. The raw, subterranean diamond is typically dull and unremarkable, often resembling a common piece of gravel or cloudy rock. Understanding what a diamond looks like in the ground requires exploring the incredible geological journey it takes, from its formation deep within the planet to its host rock.
The Extreme Conditions of Diamond Formation
A diamond begins its existence under conditions of extreme pressure and heat, far beneath the Earth’s crust in the mantle. This unique environment is known as the diamond stability field, a narrow window where carbon atoms organize into the dense, tetrahedral structure of diamond instead of graphite. The formation process requires temperatures ranging between 900°C and 1,300°C, combined with massive pressures of 4.5 to 6 gigapascals. These conditions are met at depths spanning 140 to 250 kilometers below the surface, specifically within the ancient, cool, and stable roots of continental plates.
The carbon atoms must remain within this deep stability field for millions, or even billions, of years to fully crystallize. The diamonds are then brought rapidly to the surface in rare, deep-source volcanic eruptions. This rapid ascent through the mantle and crust is the only way the diamonds avoid being converted back into graphite as the pressure decreases. The explosive magma creates vertical conduits, or pipes, that serve as the transport mechanism, carrying the diamonds and other mantle material up to the crust.
Appearance of the Rough Diamond Crystal
The raw diamond crystal, when first extracted, displays a characteristic shape dictated by its internal atomic structure. The most frequent natural shape is the octahedron (two pyramids joined base-to-base), though dodecahedrons and cubic forms are also common. Unlike the perfectly smooth, faceted surfaces of a cut gem, the rough crystal often has a natural, unrefined texture. This surface can exhibit features like subtle growth triangles or etch pits, which are small depressions created by chemical reactions during its ascent.
Many rough diamonds possess a cloudy or opaque appearance, lacking the brilliance people associate with the stone. They are frequently found with various tints, often appearing yellow, brown, or green, due to trace elements or structural imperfections. The surface of a newly mined stone can also display a waxy or greasy luster, which further diminishes any inherent sparkle. It takes the precise skill of a cutter and polisher to remove the rough exterior, reveal the clarity, and create the facets that generate fire and sparkle. The vast majority of rough stones are not gem-quality and are instead used for industrial purposes due to their exceptional hardness.
Host Rocks and Deposit Environments
The primary geological context for finding diamonds is within the cooled volcanic rock that formed the deep-source eruption conduits. These host rocks are primarily known as kimberlite and, less commonly, lamproite. Kimberlite is an igneous rock, often bluish-green or gray, that forms a carrot-shaped pipe structure extending deep into the Earth. The diamonds are embedded within this hard rock matrix, requiring significant mining and processing to separate the crystals from the surrounding material.
An alternative environment for finding diamonds is in secondary, or alluvial, deposits, which are the result of millions of years of erosion. Over geological time, the original kimberlite and lamproite pipes were weathered away, and the liberated diamonds were carried by rivers and streams. Because diamonds are extremely dense and chemically resistant, they settled in riverbeds, coastal areas, and gravel deposits far from their primary source. Diamonds recovered from these alluvial environments are often slightly more rounded or worn down due to the tumbling action of water transport. This natural sorting process sometimes results in a higher proportion of durable, gem-quality stones in alluvial environments compared to the pipes themselves.