When set into jewelry, a diamond’s brilliant sparkle results from expert cutting and polishing. Before this transformation, a “diamond rock,” or rough diamond, bears little resemblance to the faceted gems people commonly imagine. In their natural state, diamonds are unpolished, often opaque crystals that look like simple, dull stones or pieces of cloudy glass. Understanding what a diamond truly looks like in nature requires examining its raw crystal structure, surface characteristics, and geological environment.
The Primary Crystal Structure
A rough diamond’s appearance is defined by its crystal system, known as the cubic system. The most common shape for a naturally forming diamond is the octahedron, which looks like two four-sided pyramids joined base-to-base, creating an eight-sided form. This shape results from the precise internal arrangement of carbon atoms.
Formation conditions vary, meaning not all rough diamonds maintain this perfect double-pyramid shape. Less common forms include the dodecahedron (12-sided shape) and the cube (six-sided crystal). During their ascent from the Earth’s mantle, many crystals undergo resorption, where part of the diamond dissolves. This process often rounds the sharp edges and corners, making even an octahedral diamond look more like a slightly distorted pebble.
Color, Clarity, and Surface Characteristics
The color of a rough diamond is usually not the colorless transparency prized in finished jewelry. Most natural diamonds contain trace elements, such as nitrogen, causing them to exhibit a pale yellow or brown hue. Stones with distinct, saturated colors like pink, blue, or green are known as “fancy colors” and are exceptionally rare.
Rough diamonds often possess a greasy or waxy surface luster when first recovered, contributing to their unremarkable appearance. Their surfaces frequently display distinct growth markings. Triangular depressions known as trigons are common features unique to diamonds that indicate the crystal’s growth pattern.
Clarity is a major factor, as most crystals contain internal imperfections called inclusions. These can be tiny foreign mineral crystals or minute structural flaws that make the diamond appear cloudy, milky, or opaque. The presence of these features causes many rough diamonds to be classified as industrial grade, often appearing dark gray or black, much like common rock.
Geological Settings Where Diamonds Are Found
Rough diamonds form under heat and pressure deep within the Earth’s mantle, approximately 90 to 120 miles below the surface. They are transported rapidly to the surface through rare volcanic eruptions, preserved in formations known as kimberlite pipes. Kimberlite is a bluish-gray to greenish-gray igneous rock matrix that fills these ancient conduits.
In this primary setting, diamonds are embedded within the kimberlite rock and must be crushed out during mining. Diamonds recovered directly from kimberlite pipes often retain sharp, geometric crystal shapes like the octahedron or dodecahedron.
The second type of deposit is alluvial, where diamonds have been eroded from their original kimberlite source and carried away by rivers. These diamonds are found in riverbeds, deltas, or along coastlines. Alluvial diamonds typically have a worn and rounded appearance due to abrasion, resembling smooth, frosted pebbles.