Diamond is a solid, crystalline form of carbon, an allotrope, meaning it is one of the distinct structural forms that carbon can take. Answering whether diamonds decay requires looking beyond their reputation for permanence, delving into the specific chemical and physical forces that affect them. At a fundamental level, diamonds do not decay under the normal conditions found on the surface of the Earth. The idea of “decay” refers to a breakdown or transformation, and the diamond’s remarkable stability makes this process exceptionally slow, though not impossible, as determined by thermodynamics and kinetics.
The Chemical Foundation of Diamond Stability
The exceptional stability of diamond is based on its unique atomic structure. Each carbon atom is bonded to four neighboring carbon atoms in a perfect, three-dimensional tetrahedral arrangement. This structure results in a dense, continuous network of atoms, which accounts for the material’s extreme hardness and resistance to chemical change.
The bonds connecting these carbon atoms are powerful \(\text{sp}^3\) covalent bonds, some of the strongest chemical bonds found in nature. These strong, directional bonds form a rigid lattice that requires a significant input of energy to break or rearrange. This structural integrity makes diamond the hardest known natural substance.
From a chemical perspective, diamond is considered a metastable material at standard temperature and pressure (STP). This means the crystalline structure of diamond is not the absolute lowest energy state for carbon at the Earth’s surface. However, the energy barrier required to transition to the true lowest-energy state is extraordinarily high. The energy difference between diamond and its more stable counterpart, graphite, is minor, approximately 2.9 kilojoules per mole. The activation energy, the energy needed to initiate the change, is massive, estimated to be between 727 and 1057 kilojoules per mole.
This enormous activation energy barrier prevents a diamond from spontaneously transforming into graphite, even though graphite is thermodynamically more stable. The atoms simply do not possess enough kinetic energy to overcome this barrier under normal conditions. This kinetic stability effectively locks the carbon atoms into the diamond structure, preserving the gem’s form over vast periods of time. This structural quality is why ancient diamonds, some billions of years old, remain unchanged when found in nature.
Diamond’s Slow Transformation to Graphite
The scientific definition of decay for a diamond involves its slow, natural transformation into graphite, the most stable form of carbon at ambient pressure. This process, known as graphitization, is the ultimate fate of diamond, but it occurs at an almost immeasurable rate under surface conditions. The immense geological age of natural diamonds demonstrates that this conversion is practically nonexistent in human timescales.
For the transformation to become noticeable, the diamond must be exposed to conditions that provide the atoms with sufficient energy to overcome the high activation barrier. In a controlled, oxygen-free environment, high temperatures are required to accelerate graphitization. The transformation rate becomes significant at temperatures around \(1700^\circ \text{C}\) to \(1800^\circ \text{C}\) under vacuum or inert gas.
Below these extreme temperatures, the conversion process is still occurring, but at an insignificant rate, even over millions of years. The carbon atoms must rearrange from their dense, three-dimensional tetrahedral structure into the layered, planar structure of graphite. This process involves breaking the strong \(\text{sp}^3\) bonds and forming weaker \(\text{sp}^2\) bonds, a demanding atomic-scale reorganization that thermal energy must drive.
In industrial and laboratory settings, the conversion to graphite can be observed and even catalyzed at lower temperatures, especially on the diamond’s surface. However, this is always an induced event, requiring sustained, elevated heat far exceeding natural environmental conditions. The fact that diamonds endure for billions of years at the Earth’s surface confirms that the slow, internal process of chemical decay is not a practical concern for any owner.
External Factors That Cause Degradation
While diamond resists internal chemical decay, it is vulnerable to rapid destruction from external forces, which is often mistaken for decay. One dramatic form of external degradation is oxidation, or burning, since diamond is a form of carbon. When exposed to oxygen at sufficiently high temperatures, a diamond will react, turning into carbon dioxide gas.
This combustion reaction begins to occur at temperatures between \(600^\circ \text{C}\) and \(700^\circ \text{C}\) in air, though the rate of oxidation increases significantly as the heat rises. This rapid, destructive chemical change is distinct from the slow, internal graphitization process because it involves a reaction with an external element, oxygen. At temperatures above \(900^\circ \text{C}\), the diamond will burn away completely in a matter of hours.
The second primary form of degradation is physical breakage, which exploits the crystal’s inherent planes of weakness. Despite being the hardest material, diamonds possess perfect octahedral cleavage along the \(\{111\}\) crystallographic planes. If a precise, sharp impact is delivered parallel to one of these four planes, the diamond can be cleaved, or split, creating a clean break rather than an irregular fracture. This vulnerability is utilized by diamond cutters but also means that a severe, specific impact can cause a diamond to break apart, which is physical destruction, not chemical decay.
Finally, while diamonds are resistant to nearly all common acids and bases, they can be degraded by highly aggressive chemical agents under extreme conditions. Specific oxidizing molten salts or highly reactive gases can etch or react with the diamond surface. However, these are specialized industrial or laboratory reagents, such as a strong ozone treatment at \(425^\circ \text{C}\), and do not represent a threat under normal household or environmental circumstances.