The common saying that “only a diamond can cut another diamond” is a precise statement of material science, not just a metaphor for strength. Hardness refers to a material’s resistance to scratching, abrasion, or permanent surface deformation. Diamond is the hardest naturally occurring substance known, meaning no other natural material possesses the physical properties necessary to abrade it. Shaping a rough diamond into a brilliant gemstone therefore requires the use of diamond itself, often in the form of specialized tools. This unique property originates from the highly organized internal arrangement of its carbon atoms.
The Atomic Structure Behind Extreme Hardness
Diamond’s extraordinary hardness stems from its unique crystal lattice structure, a highly stable and rigid three-dimensional network. Each carbon atom is covalently bonded to four other carbon atoms in a perfect tetrahedral arrangement. This distributes the bonds evenly in space, forming an extremely dense and uniform structure throughout the crystal.
Covalent bonds are formed by sharing electrons, and the carbon-carbon bonds within the diamond lattice are among the strongest known in nature. Since every carbon atom is locked into this arrangement with four neighbors, the entire crystal acts as a single, giant molecule. This atomic scaffolding leaves virtually no weak points for an external force to exploit or for the structure to cleave or deform.
The dense packing of atoms minimizes the space between them, making it nearly impossible for a foreign material to penetrate the surface and cause a scratch. Any attempt by a softer material to abrade the diamond results in the bonds within the softer material breaking first. The uniformity of the crystal structure is the physical reason why diamond is so resistant to scratching, requiring a material with an equal or greater bond strength to achieve any abrasive effect.
The Process of Shaping and Polishing Diamond
Specialized methods must be employed to transform a rough stone into a finished gem. The initial stage of cutting, which separates a large rough diamond into smaller pieces, is achieved through sawing or cleaving. Sawing utilizes rotating blades coated with a mixture of oil and fine diamond powder, effectively using tiny diamonds to wear down the stone.
Cleaving relies on the diamond’s natural internal planes of weakness. A precise notch is cut into the diamond, usually with a laser or a diamond edge, and then a sharp impact with a non-diamond tool can split the stone along this specific plane. After this initial separation, the next stage, known as bruting, begins to shape the diamond into its final rounded outline.
Bruting is historically done by mounting two diamonds on spinning axles and grinding them against one another into a circular shape. Modern techniques for polishing and faceting involve pressing the diamond against a rotating cast-iron wheel called a scaife. The wheel’s surface is impregnated with a paste made of diamond dust, ensuring the only abrasive agent touching the stone is its own material.
While mechanical abrasion requires diamond tools, advanced methods like high-powered lasers are used for precision cutting. These lasers do not physically cut the stone; they heat the carbon to such an extreme temperature that it vaporizes directly from a solid into a gas. This method bypasses mechanical hardness by fundamentally changing the material’s state, but traditional shaping still relies on diamond against diamond.
Testing Diamond Hardness Against Other Materials
Diamond’s status as the hardest natural substance is quantified using the Mohs scale of mineral hardness, an ordinal scale ranking materials based on scratch resistance. Diamond sits at the maximum value of 10, indicating it can scratch every other material on the scale. The difference between a rating of 9, such as corundum (ruby and sapphire), and diamond at 10 is not linear; diamond is many times harder than corundum in absolute terms.
The Mohs scale test involves trying to scratch one mineral with another. Since no natural mineral can scratch diamond, it remains the benchmark for natural hardness. While diamond is the hardest naturally occurring material, scientists have developed synthetic materials that approach or sometimes exceed its scratch resistance. Materials like cubic boron nitride (c-BN) are close rivals, and aggregated diamond nanorods are experimentally harder than natural diamond.
These synthetic superhard materials often require extreme pressure and temperature to synthesize and are typically used only in highly specialized industrial applications. For practical purposes, diamond remains the standard for extreme abrasion resistance. Its unique combination of a perfect crystal structure and the strongest natural covalent bonds ensures it must be cut and polished by its own kind.