Diamonds are the hardest substance on Earth, leading to the question of whether this material can damage another identical material. The answer is definitively yes: a diamond can scratch another diamond. The mechanism behind this seemingly contradictory fact is not a failure of its hardness, but a subtle variation in the atomic structure of the diamond crystal itself.
Understanding Diamond Hardness and the Mohs Scale
Hardness, in a scientific context, is defined as a material’s resistance to permanent surface deformation, such as scratching or abrasion. The widely known Mohs scale of mineral hardness measures this property on an ordinal scale from 1 to 10. Diamond occupies the top rank at 10, meaning it can scratch all other naturally occurring minerals, including corundum (ruby and sapphire) at a 9.
The Mohs scale measures relative, not absolute, hardness. The jump in actual hardness from corundum at 9 to diamond at 10 is far greater than the difference between any other two steps on the scale. Diamond is estimated to be approximately four times harder than corundum.
Since the Mohs scale is relative, the rating of 10 means a diamond is harder than everything else. It does not mean all diamonds are equally hard or that they cannot be abraded by something with the same rating. The scale provides a general measure of surface resistance, but it does not account for the microscopic structural differences found within a single diamond crystal.
Anisotropy: The Directional Vulnerability
One diamond can scratch another due to anisotropy, meaning the physical properties of the material are directionally dependent. A diamond crystal is a repeating lattice of carbon atoms, but the density of the carbon-carbon bonds varies depending on the crystallographic direction. This variation creates “hard” and “soft” directions on the diamond’s surface.
For example, the octahedral faces of the diamond crystal, where atoms are most densely packed, exhibit the highest resistance to abrasion. Conversely, other faces, such as the cube faces, offer less resistance to scratching. An abrasive force applied by a diamond’s hard direction against a softer direction on another diamond will result in a scratch.
This directional difference is the principle used in the diamond industry for cutting and polishing. Diamond cutters use diamond powder to slowly abrade a larger diamond into its final faceted shape. Without this anisotropic property, shaping a diamond would be nearly impossible. The action of one diamond scratching another is therefore a controlled exploitation of its structural vulnerability.
Cleavage and Fracture: Protecting Your Diamonds
While scratching is a concern when diamonds rub against each other, the more common and significant risk of damage comes from a different physical property: cleavage. Cleavage is the tendency of a crystalline material to split along specific planes of weakness where the atomic bonds are less strong. Diamonds possess four perfect cleavage planes, which align with the faces of the octahedron shape.
Cleavage contrasts with fracture, which is an irregular break that does not follow a specific crystal plane. Despite being the hardest known material, a diamond is brittle, meaning a sharp, sudden impact in the wrong direction can cause it to cleave or chip. This is why a diamond, though scratch-resistant, is susceptible to damage if struck against a hard surface.
Most damage to diamond jewelry occurs not from a diamond scratching itself, but from chipping due to impact on a vulnerable area, such as the girdle or the pointed tip of a pear or marquise cut. To protect a diamond, the simplest action is to store each piece of jewelry separately so that the hard edges of one stone cannot repeatedly scrape the surface of another. Choosing protective settings, such as a bezel or a more robust prong design, can also help shield the vulnerable edges from accidental impact.