Diamond is the strongest natural material, measured by its compressive strength, or resistance to crushing force. This exceptional resistance to being compressed or deformed is a direct consequence of its unique internal structure. Understanding how much pressure a diamond can tolerate involves exploring the atomic forces that bind it together and the specialized scientific tools developed to test material limits.
The Atomic Structure Behind Diamond’s Strength
Diamond’s extraordinary strength originates from the arrangement and bonding of its carbon atoms. Each carbon atom is joined to four other carbon atoms in a perfect three-dimensional crystalline lattice known as a tetrahedral structure. This geometry is achieved through a specific type of electron sharing called sp3 hybridization.
The sp3 covalent bonds are among the strongest chemical bonds found in nature, creating a rigid, continuous network. This structure effectively makes a single diamond crystal a giant molecule, distributing stress evenly across the entire lattice. The dense, three-dimensional interlocking of these bonds resists any force attempting to push the atoms closer together.
Measuring Pressure Using the Diamond Anvil Cell
To test materials under extreme conditions, scientists use a specialized device called the Diamond Anvil Cell (DAC). The DAC is a simple but ingenious apparatus that generates ultra-high pressures by forcing the tiny, flat tips—or culets—of two opposing diamonds against a sample. A modest amount of force applied to the wide, flat base of the diamond is concentrated onto the minuscule culet face, generating tremendous pressure on the sample sandwiched between them.
Diamonds are the only material capable of serving as the anvils because they are the only known substance that can withstand the resulting pressure without deforming or failing. The sample, often enclosed in a metal gasket, can be examined visually or with X-rays directly through the transparent diamonds. This setup allows researchers to recreate and study the intense pressure environments found deep within the interior of Earth and other planets. Pressure inside the DAC is commonly measured by observing the shift in the fluorescent light emitted by a tiny ruby chip placed alongside the sample.
The Maximum Pressure Diamond Can Withstand
The practical limit of pressure a diamond anvil can generate is staggeringly high, with record-setting experiments achieving up to 770 gigapascals (GPa). However, the exact pressure at which the diamond itself fails depends heavily on the quality and orientation of the specific crystal being used.
Bulk diamond, in its natural form, has a compressive strength measured up to approximately 60 GPa. Newer experiments using micro- or nano-sized diamond structures, which contain fewer internal flaws, have demonstrated far greater strength. Depending on the crystal orientation, these minuscule structures can exhibit compressive strengths ranging from 125 GPa to over 240 GPa before yielding.
The failure of the diamond anvils in the DAC is not typically due to uniform hydrostatic pressure, but rather the immense shear forces and defects present at the tiny contact point. The ideal theoretical strength of a flawless diamond lattice is predicted to be even higher, calculated around 225 GPa under uniaxial tension. Achieving the highest pressures requires virtually perfect diamonds, with specific crystal orientations providing the greatest resistance to shear stress.
Phase Transitions When Diamond Fails
When the pressure applied to a diamond anvil exceeds its limit, the material does not simply shatter into pieces under pure compression. Instead, the extreme, non-hydrostatic stress—often combined with high shear forces—induces a structural change in the carbon atoms. This failure manifests as a phase transition, where the highly ordered cubic diamond structure begins to transform.
Under these conditions, particularly at the surface of the culet faces, the atoms can rearrange into a different form of carbon. This transformation is commonly observed as a process of graphitization, where the diamond structure begins to revert to its more stable, layered form, graphite. The resulting transformation highlights that even the strongest material in nature is only metastable, with its ultimate failure being a fundamental change in the bonding and arrangement of its atoms.