The diamond is the hardest naturally occurring material on Earth. Hardness is defined as a material’s resistance to surface scratching, which is distinct from toughness, or resistance to breaking or chipping. While diamonds are nearly impervious to scratching by most substances, they are not indestructible. The ability to scratch a diamond depends entirely on the material doing the scratching.
Understanding Extreme Hardness
The diamond’s extraordinary resistance to scratching comes from its internal atomic architecture. Each carbon atom is locked in a tight, three-dimensional tetrahedral lattice, connected by immensely strong covalent bonds. This dense, rigid arrangement makes it extremely difficult to displace surface atoms, which is the action required for a scratch to occur.
The mineral’s hardness is measured using the Mohs scale, a ranking system from 1 to 10 that indicates a mineral’s relative ability to scratch another. Diamond sits alone at the peak of this scale with a ranking of 10. The difference in absolute hardness between diamond and the next hardest mineral, corundum (Mohs 9), is not linear; diamond is many times harder than corundum. This significant gap illustrates why virtually all other materials fail to leave a mark on a diamond’s surface.
How Diamonds Act as Abrasives
Due to its supreme hardness, the diamond is used extensively as an abrasive, easily scratching and cutting every material below it on the Mohs scale. Industrial applications leverage this property by using small diamond particles embedded in tools like saw blades, drill bits, and grinding wheels. These diamond-tipped tools are essential for shaping and cutting ultra-hard materials such as ceramics, glass, and tungsten carbide.
In a jewelry context, a diamond can readily scratch the precious metals of its own setting, such as gold, platinum, or silver, which are much softer. It can also scratch other popular gemstones, including quartz, topaz, and even rubies or sapphires. For this reason, a diamond left loose in a jewelry box can cause significant damage to other pieces.
What Can Actually Scratch a Diamond
Only a substance with the same or greater hardness can affect a diamond’s surface. Therefore, the only material that can scratch a diamond is another diamond. This principle is used in the gem-cutting process, where diamond-tipped tools and diamond powder are used to cut and polish diamonds themselves.
A diamond’s hardness is not uniform in every direction, a property known as directional hardness. The carbon bonds are slightly weaker along certain crystalline planes, allowing a diamond cutter to orient a second diamond to exploit these softer directions. This directional variation allows one diamond to effectively scratch or cut another.
Diamonds possess a vulnerability known as perfect cleavage, meaning they can cleave, chip, or fracture internally if they receive a sharp, forceful blow along one of their natural planes of weakness. While the diamond resists scratching from all but its own kind, it is far from being the toughest material and can be damaged by impact.
Protecting Your Diamond Jewelry
Understanding the diamond’s properties translates directly into proper care for diamond jewelry. Since only another diamond can cause surface abrasion, the primary rule for protection is to ensure they are never stored together. Placing diamond jewelry in a single compartment allows them to rub against each other, potentially causing damage.
Wearers should also avoid activities where the stone might be struck with a hard, sudden impact, due to the risk of chipping along cleavage planes. Removing a diamond ring before heavy labor or exercise can prevent a fracture. Storing each piece separately in a fabric-lined box or individual pouch is the best way to maintain the stone’s condition.