Diamond is widely recognized for its extreme hardness. While exceptionally hard, scientific advancements and specific definitions show it is not always the absolute hardest material in all contexts. This common belief, accurate for naturally occurring minerals, requires understanding how hardness is defined and measured. The answer involves distinctions between different types of hardness and the emergence of novel materials.
Measuring Mineral Hardness
Mineral hardness refers to a material’s resistance to scratching or abrasion. The Mohs scale of mineral hardness is the most widely recognized method for gauging this property. Developed by Friedrich Mohs in 1812, this empirical scale ranks minerals from 1 to 10 based on their ability to scratch one another. Talc sits at 1, and diamond, known for its scratch resistance, is at 10.
A harder mineral will scratch a softer one, but not vice versa. For instance, quartz (Mohs 7) can scratch feldspar (6), but not topaz (8). While the Mohs scale provides a useful relative ranking, it is not linear; the difference in absolute hardness between 9 and 10 is far greater than between 1 and 2. For more precise measurements, especially in materials science, laboratory tests like the Vickers or Knoop hardness tests are employed. These methods measure indentation hardness, assessing a material’s resistance to plastic deformation from a defined force.
The Source of Diamond’s Hardness
Diamond’s remarkable hardness stems from its unique atomic structure and bonding. It consists solely of carbon atoms arranged in a stable, tetrahedral lattice. Each carbon atom is covalently bonded to four others, forming a strong, interconnected network. These covalent bonds are among the strongest known in nature, requiring significant energy to break.
This tightly packed, three-dimensional crystalline structure prevents atoms from easily sliding past one another. The strong, directional bonds resist deformation and scratching, making diamond highly resistant to abrasive wear. Its designation as a 10 on the Mohs scale reflects its superior scratch resistance among naturally occurring minerals. The uniformity and strength of these bonds throughout the crystal contribute to its consistent hardness.
Materials Harder Than Diamond
While diamond holds the top spot on the Mohs scale for natural minerals, scientific research has led to the discovery and synthesis of materials that challenge or surpass its hardness under specific conditions. One such material is wurtzite boron nitride (w-BN), a synthetic compound with a structure similar to diamond but composed of boron and nitrogen atoms. Theoretical calculations and experimental observations suggest w-BN can exhibit superior indentation hardness to diamond, particularly under high pressures. Its unique atomic arrangement contributes to its enhanced resistance to compression.
Another notable example is aggregated diamond nanorods (ADNRs), also known as ultrahard fullerite. These synthetic materials are formed by compressing and heating C60 fullerene molecules. ADNRs consist of interconnected diamond nanorods, creating a bulk material with higher resistance to indentation than single-crystal diamond. Their unique microstructure, with nanoscale diamond grains, distributes stress more effectively, enhancing overall hardness. These synthetic advancements highlight that while natural diamond remains very hard, human ingenuity can engineer materials with even harder properties.