The diamond is the hardest naturally occurring substance on Earth, scoring a perfect 10 on the Mohs scale of mineral hardness. This supreme durability, derived from its dense, tetrahedrally bonded carbon lattice, presents a unique challenge for transforming it into a brilliant gemstone. Unlike softer materials, a diamond can only be worked by a material of equal or greater hardness. This physical constraint means the process of cutting and polishing a rough diamond is a sophisticated blend of traditional craftsmanship and advanced industrial technology. The diamond itself provides the answer to what cuts a diamond.
The Primary Tool: Cutting Diamond With Diamond
The foundational method for modifying a rough diamond relies on the principle of abrasion, specifically using diamond material against diamond material. Early in the cutting process, preliminary separation is often performed using a rotating metal disk coated with a mixture of diamond powder and oil. These specialized blades are typically made of phosphor-bronze or a similar metal alloy designed to hold the abrasive diamond grit securely. The disk spins at high velocity, and the embedded diamond particles physically wear away the rough stone.
This abrasive technique is effective because the diamond used as the tool is randomly oriented, presenting different crystalline faces to the diamond being cut. A diamond’s hardness is anisotropic, meaning it varies depending on the direction of the cut relative to its internal atomic structure. Expert cutters must first analyze the rough stone’s crystalline orientation to locate the “soft” directions, where the material is less resistant to abrasion. The diamond saw then exploits these softer planes, allowing the tool’s diamond dust to grind through the host stone.
Modern Advancements: Laser Technology and Specialized Saws
Newer technology has introduced non-abrasive methods, drastically improving efficiency and precision, particularly for complex cuts. High-powered industrial lasers, such as Nd:YAG or femtosecond pulsed lasers, are now used extensively for sawing and cleaving rough stones. This method works through thermal and chemical transformation, not physical contact or abrasion.
The focused laser beam introduces intense, localized heat that causes the diamond’s carbon lattice to convert into graphite. Since graphite is less stable than diamond, the heat causes the newly formed graphite to vaporize or sublime. This process of laser-induced graphitization creates a clean, narrow cut with minimal material waste, contrasting with abrasive methods that produce dust. Another modern advancement is the use of diamond wire saws, which consist of a high-tensile steel cable threaded with diamond-impregnated beads. While still an abrasive technology, these saws are used primarily for extremely large stones or in controlled industrial settings to make precise, deep cuts. The wire operates in a continuous loop under tension, allowing for controlled slicing that minimizes vibration and damage.
Precision Shaping: Bruting, Faceting, and Polishing
After initial sawing, the stone undergoes bruting, which is the process of shaping the girdle, or the circular outer edge of the diamond. This stage involves mounting two diamonds onto spinning axles and rubbing them against each other, one acting as the tool and the other as the workpiece. Modern bruting is largely automated using sophisticated lathes, but the principle remains the same: diamond against diamond creates the preliminary round shape.
The most defining and time-intensive stage is faceting and polishing, which gives the finished gem its characteristic brilliance. This intricate work is performed on a rapidly rotating cast iron wheel, historically known as a “scaif.” The scaif is charged with a specialized slurry—a paste composed of fine diamond powder suspended in a carrier fluid, such as oil or a proprietary chemical blend. The diamond powder embedded in the porous surface of the scaif performs the final abrasion.
The diamond being polished is held in a specialized mechanical clamp, called a polishing tang, which allows the cutter to precisely control the angle at which the stone meets the scaif. The cutter must meticulously create the typically 57 or 58 facets of a brilliant cut diamond, ensuring their angles and alignment maximize light reflection. The quality and size of the diamond powder used in the slurry is progressively reduced throughout the process, moving from coarser grit for initial faceting to ultra-fine powders for the final, mirror-like polish.