The image of a diamond is typically one of dazzling brilliance and precise geometry, but a rough diamond when first unearthed bears little resemblance to the polished gemstones seen in jewelry. Instead of a sparkling, faceted jewel, the newly discovered stone is a raw crystal with an often dull and unremarkable appearance. It is a natural mineral specimen that must undergo cutting and polishing to unlock the famed optical properties of a finished diamond.
The Natural Crystalline Shape
The structure of a rough diamond is dictated by its atomic arrangement, placing it within the cubic crystal system. The most frequently observed geometric form is the octahedron, which appears as two four-sided pyramids joined base-to-base, resulting in a shape with eight triangular faces. Other shapes include the dodecahedron, which has 12 rhombus-shaped faces, and the less common cubic form, defined by six square faces.
These ideal shapes are often modified due to varying conditions during their formation and ascent from the Earth’s mantle. Many rough crystals exhibit a distorted shape or are “twinned,” where two crystals grow together with a mirror-image relationship, creating a triangular and often flattened shape known as a macle. Furthermore, the high-pressure, high-temperature environment can cause partial dissolution, or resorption, which rounds the edges and corners of the crystal, transforming a sharp octahedron into a more spherical or irregular form.
Surface Characteristics and Luster
The external texture of a rough diamond is a primary indicator of its raw state, lacking the intense reflective quality of a cut stone. Freshly recovered diamonds often display a greasy, waxy, or opaque gum-like coating on their surface, known in the industry as “nyf.” This outer layer significantly reduces the stone’s transparency and causes the rough, unpolished luster, which aids in field identification.
The natural coloration of a rough diamond is rarely the pure white of a colorless gem, frequently appearing in shades of yellowish, brownish, or opaque gray. Close inspection of the crystal faces may reveal specific growth features, such as trigons—small, triangular depressions or elevations on the octahedral faces. These trigons result from the diamond’s growth or dissolution process and are unique markers of its identity. The overall appearance is more akin to a common piece of gravel or a heavily frosted pebble rather than a transparent crystal.
Geological Settings of Discovery
The context in which a diamond is found largely influences its immediate appearance and surrounding material. Diamonds are delivered to the Earth’s surface through explosive volcanic eruptions originating deep in the mantle, where they are embedded in primary source rocks. These rocks are typically kimberlite or lamproite, which form carrot-shaped vertical structures known as pipes.
In these primary deposits, the rough diamond is recovered directly from the host rock, often a dark, dense matrix that must be crushed to free the crystal. Conversely, in secondary or alluvial deposits, the diamonds have been naturally eroded from their original source and transported by water over vast distances. Here, they are found mixed with river gravel, sand, and other sediments, a process that further rounds and smooths their surfaces due to mechanical tumbling.
Key Properties Used for Identification
Once recovered, certain physical properties immediately distinguish a rough diamond from other minerals. Diamond is the hardest naturally occurring substance, registering a 10 on the Mohs scale, meaning it can scratch nearly any other mineral it encounters. This extreme hardness is a reliable initial test, though the stone is not invincible; it possesses perfect cleavage, meaning a sharp blow along one of its four cleavage planes can cause it to fracture.
Another telltale feature is its high density, or specific gravity, which averages around 3.51 grams per cubic centimeter, making it feel notably heavier than a piece of quartz of similar size. Specialized tools utilize the diamond’s exceptional thermal conductivity, a property superior to most other materials, to quickly verify its authenticity in the field. Internally, rough diamonds frequently contain tiny mineral inclusions or exhibit stress lines related to the crystal’s structure, providing further confirmation of its identity to an experienced sorter.