Diamonds are a crystalline form of pure carbon, forged deep within the Earth’s mantle under immense pressure and high temperatures. These rare structures are not typically found as sparkling, loose gems, but are instead embedded within a protective host rock. The appearance of a rough diamond in its natural state is remarkably different from the brilliant, cut stone seen in jewelry. Finding one requires knowing the characteristics of the host rock and how to distinguish a rough diamond from a common pebble.
The Geological Home of Diamonds
The journey of a diamond to the Earth’s surface begins hundreds of miles down. Diamonds are carried up through rapid, violent volcanic eruptions, which form narrow, vertical structures in the crust known as volcanic pipes. The most common primary source rock is Kimberlite, a dark, ultramafic igneous rock named after the South African town where it was first studied.
Kimberlite often appears bluish-green but weathers to a reddish-brown color near the surface due to oxidation. It is a dense matrix containing fragments of other rocks and minerals, notably olivine, garnet, and phlogopite mica. A less common host rock is Lamproite, which is the primary source in regions like Western Australia. Both Kimberlite and Lamproite transport the diamonds quickly, preventing them from dissolving back into graphite.
Visual Characteristics of Diamonds in the Matrix
When encased in its host rock, a diamond rarely exhibits the fiery brilliance of a finished gemstone. A rough diamond’s shape is determined by its internal crystal structure, often resulting in geometric forms. These forms include an octahedron, which resembles two pyramids joined at their bases, or a dodecahedron. These distinct geometric shapes are a primary visual cue that a specimen might be a diamond.
The surface texture of a rough diamond is typically dull and unpolished, often appearing rough, pitted, or etched from its turbulent journey. This unrefined exterior gives the diamond a greasy, oily, or waxy luster, unlike the glassy appearance of many other minerals. Most rough diamonds are not perfectly clear; they often exhibit colors like yellow, brown, or gray, or appear translucent to opaque due to natural impurities.
Minerals Often Mistaken for Diamonds
The probability of mistaking a common mineral for a rough diamond is high, as many minerals in diamond-bearing areas look similar to the untrained eye. Quartz, composed of silicon dioxide, is one of the most abundant minerals and is frequently confused with diamonds. Unlike a diamond’s cubic structure, quartz crystals form in hexagonal shapes and have a significantly lower Mohs hardness rating. Quartz also has a much lighter specific gravity, meaning it will feel less dense than a diamond of similar size.
Corundum, the mineral group that includes rubies and sapphires, is the second-hardest natural mineral after diamond. Corundum crystals are typically more colorful and exhibit a trigonal crystal structure, which differs from the cubic system of a diamond. The way light interacts with corundum also produces a less intense reflection than a diamond, even in its rough state.
Simple Tests to Confirm a Potential Diamond
Once extracted from the host rock, a few simple tests can help gauge a suspected diamond’s authenticity. The Mohs hardness scale ranks diamond at 10, meaning it can scratch nearly any other material. A simple scratch test against common glass or a steel file can indicate if the specimen is hard enough to be a diamond. However, this test can damage the stone and is not definitive.
The “fog test” relies on a diamond’s exceptional thermal conductivity. When a person breathes lightly onto the stone, a real diamond disperses the heat almost instantly, causing the fog to clear immediately. If the fog lingers on the surface for a few seconds, the stone is likely an imitation or a different mineral with lower thermal properties. A final test uses density: if the stone is loose, dropping it into water will cause a genuine diamond to sink quickly, whereas lower-density simulants may float or hover.