Raw diamonds, the natural form of the stone before cutting and polishing, appear distinctly different from the brilliant gemstones commonly seen in jewelry. They are typically dull, rough, and may even resemble an ordinary pebble or a piece of cloudy glass. This difference is due to the absence of precisely cut facets that create the familiar light reflection and brilliance. Raw diamonds are found embedded in specific types of ancient volcanic rock, which complicates their identification for the untrained eye.
The Geological Context: Diamond Host Rocks
Diamonds are created under immense pressure and heat deep within the Earth’s mantle, over 150 kilometers below the surface. They are transported to the crust through rare, violent volcanic eruptions that form structures known as pipes. The primary host rocks for diamonds are kimberlite and, less frequently, lamproite.
Kimberlite is an igneous rock named after the South African town of Kimberley. This rock often presents a dark, dense appearance, sometimes appearing blue-gray or green-gray, and is rich in minerals like olivine and carbonate. Weathered kimberlite near the surface is known as “yellow ground,” while the unweathered rock below is called “blue ground.” Lamproite is a less common host rock, rich in potassium and magnesium.
Visual Appearance of Uncut Diamond Crystals
Once separated from the host rock, a raw diamond crystal has a distinct appearance. The most common natural shape is the octahedron, resembling two four-sided pyramids joined at their bases. Other shapes include the dodecahedron, which has twelve faces, and the cube, though irregular or fragmented shapes are also common.
The surfaces of these uncut crystals are often rough, uneven, and may display natural growth marks, such as triangular etch pits called trigons. Raw diamonds are frequently cloudy, translucent, or opaque due to internal flaws or inclusions, rarely possessing the clear, colorless look of a finished gem. Their color is often tinted yellow, brown, or gray, though rare finds can include pink, blue, or green stones. Instead of a brilliant sparkle, a raw diamond exhibits a unique, slightly reflective appearance described as a “greasy” or “adamantine” luster.
Identifying Diamonds Within the Rock Matrix
Spotting a diamond still encased in the host rock is extremely difficult because the crystal is often small, dull, and blends in with the dark, heavy matrix. When embedded, the diamond appears as a small, slightly translucent or opaque inclusion within the surrounding kimberlite or lamproite. The crystal’s rough, unpolished surface and lack of brilliance make it nearly invisible to the casual observer.
Prospectors rely heavily on specific “indicator minerals” that share the same deep-Earth origin but are found in much greater abundance than diamonds. These minerals act as visual clues to the presence of a diamond-bearing pipe. Important indicator minerals include distinctive, brightly colored garnets (especially deep-red pyrope), chromium diopside (a bright green pyroxene), and black ilmenite. Finding concentrations of these dense, durable minerals in stream sediments or soil is often the first step in tracing a path back to the source rock.
How to Distinguish Raw Diamonds from Look-Alikes
Many common minerals, such as quartz, calcite, and some types of garnet or spinel, can be mistaken for a raw diamond due to a similar crystal shape or general appearance. Quartz crystals, for example, are often hexagonal and possess a glassy luster, which differs from the diamond’s greasy shine. Calcite and quartz are also far less dense than a diamond, which means a genuine diamond will feel heavier than a similar-sized imitation.
The most definitive property of a diamond is its hardness, ranking as a 10 on the Mohs scale, making it the hardest natural substance. A genuine raw diamond can scratch nearly any other mineral, including corundum (a 9 on the Mohs scale), while the diamond itself remains unscratched. Diamonds also have an exceptional ability to disperse heat rapidly, a property measured using specialized thermal conductivity testers. While these simple tests offer strong evidence, only professional gemological testing can provide absolute confirmation.