Diamonds are a mineral composed of pure carbon, crystallized under immense heat and pressure deep within the Earth’s mantle. Their formation requires conditions that exist only in specific, ancient regions of the continental crust, making them exceptionally rare on the surface. The search for a diamond is fundamentally a search for the unique materials associated with its formation and transport.
The Geological Origins of Diamonds
The journey of a diamond begins over 150 kilometers below the Earth’s surface, in the mantle, where temperatures exceed 1,000 degrees Celsius and pressures are enormous. Diamonds are brought to the surface through rare, violent volcanic eruptions, which create vertical, carrot-shaped structures known as kimberlite or lamproite pipes. These pipes represent the primary source rock for virtually all natural diamonds found globally, embedded within the ancient volcanic material.
Over millions of years, weathering and erosion break down these primary kimberlite and lamproite pipes. This process releases the durable diamonds from their host rock, allowing them to be carried away by rivers and streams. These water-transported accumulations are called secondary, or alluvial, deposits.
Secondary deposits can be found far from the original pipe, often concentrated in riverbeds, floodplains, or along ancient coastlines, as diamonds are dense and resistant to abrasion. While diamonds from alluvial deposits are often higher in quality because flawed stones are destroyed during transport, primary pipes still account for the majority of the world’s diamond production due to the sheer volume of rock they contain.
Identifying Diamond Indicator Minerals
A direct search for diamonds is often impractical, so prospectors instead look for diamond indicator minerals (DIMs) that were brought up from the mantle alongside the diamonds. These minerals are more common than diamonds and act as a telltale sign of a nearby kimberlite or lamproite pipe. The classic suite of indicators includes specific types of garnet, ilmenite, and chromite.
The most sought-after indicator is the chrome-pyrope garnet, which often appears as a vivid purple-red or deep orange-red crystal. These garnets, especially the chemically distinct G10 variety, signify a host rock that originated in the high-pressure environment where diamonds form. Picroilmenite, a magnesium-rich variety of ilmenite, is another common indicator, appearing as a dense, jet-black mineral with a distinct metallic luster.
Chromite, a dense, black oxide mineral, is also examined for its chemical composition, which signals proximity to a diamond-bearing source. Because these minerals are much heavier than common rock, they tend to accumulate in the same stream sediments as diamonds. Finding a concentration of these distinctive indicator minerals suggests a prospector is getting closer to the primary source pipe.
Small-Scale Prospecting and Recovery Techniques
For the individual prospector, small-scale diamond recovery focuses on alluvial deposits using the principle of gravity separation. Diamonds have a high specific gravity, meaning they are significantly denser than most river gravels and sand. Stream sampling involves collecting material from areas where heavy minerals naturally settle, such as behind boulders, in bedrock crevices, or on the inside bends of a river.
The simplest recovery technique is panning, which uses a circular pan to wash away lighter materials. A prospector immerses the pan in water, agitates the contents to allow the heavy minerals to settle to the bottom, and gently washes the lighter sediment over the rim. This process is repeated until only a small amount of heavy mineral concentrate, often referred to as “black sand,” remains in the pan.
For larger volumes of material, a sluice box can be used, which is essentially an extended channel lined with riffles or small barriers. As water and sediment flow through the sluice, the heavy minerals, including any diamonds, are caught in the riffles while the lighter material is flushed out. Techniques like jigging (using a pulsating water column) or advanced dense medium separation (DMS) can further refine the heavy mineral concentrate before it is carefully inspected for diamonds.
Recognizing a Rough Diamond
An uncut, rough diamond will rarely look like the faceted, sparkling stones seen in jewelry, often having a greasy or waxy appearance from a thin surface film. These natural crystals belong to the isometric system, meaning they typically form distinct shapes like octahedrons, which resemble two pyramids joined at the base, or dodecahedrons.
Diamond’s physical properties provide the most reliable confirmation of a find. Diamond is the hardest natural substance, scoring a 10 on the Mohs scale, meaning it can scratch nearly all other minerals. Furthermore, diamond has a high specific gravity of 3.52, making it noticeably heavier than similar-looking minerals like quartz, which has a specific gravity of around 2.6.
Testing for hardness is not always definitive, as a diamond is brittle and can cleave along specific planes if struck sharply. Therefore, a combination of characteristics—the unique crystal shape, the high density, and the ability to scratch a known hard material like corundum (Mohs 9)—is used to confirm the identity of a potential rough stone.