The hardest mineral on the Mohs scale is the diamond, which holds the maximum value of 10. Mineral hardness is defined as a material’s resistance to permanent deformation, particularly its ability to resist scratching. Diamond’s top placement on this scale highlights its unparalleled scratch resistance among naturally occurring substances. This explanation details the scale, the structure that gives diamond its supreme ranking, and the modern materials that challenge its title.
Understanding the Mohs Hardness Scale
The Mohs scale is a qualitative system developed in 1812 by German mineralogist Friedrich Mohs. The scale is not based on complex machinery but on a simple, foundational principle: a harder material will visibly scratch a softer material, but a softer material cannot scratch a harder one. This method allows for a quick, comparative assessment of scratch resistance in the field.
The scale ranges from 1 to 10, using ten specific reference minerals as anchor points for the different levels of hardness. These reference minerals are:
- Talc (1)
- Gypsum (2)
- Calcite (3)
- Fluorite (4)
- Apatite (5)
- Orthoclase feldspar (6)
- Quartz (7)
- Topaz (8)
- Corundum (9)
- Diamond (10)
The Mohs scale is relative, meaning the numerical intervals are not arithmetically equal or linear. The difference in absolute hardness between any two consecutive numbers is not the same across the scale. Steps on the lower end represent smaller differences in actual hardness than the steps toward the top.
Diamond: The Apex of Mineral Hardness
Diamond earns its position at 10 because of its internal atomic structure. It is an allotrope of carbon where each carbon atom is covalently bonded to four neighboring carbon atoms. This arrangement forms a dense, three-dimensional, tetrahedral lattice known as the diamond cubic structure.
The strength of these four-way covalent bonds throughout the crystal lattice results in structural rigidity, making it highly resistant to scratching. The leap from corundum (9) to diamond (10) is substantial. Corundum, which includes both ruby and sapphire, is roughly four times softer than diamond on an absolute hardness measurement.
As the hardest known naturally occurring mineral, diamond is used extensively in industrial applications like cutting, drilling, and polishing tools. Its ability to maintain a pristine surface is why it is highly valued as a gemstone, as only another diamond can scratch or wear down its polished facets.
Absolute Hardness and Synthetic Competitors
While the Mohs scale is practical for mineral identification, it is not used for precision engineering due to its qualitative nature. Scientists use more quantitative measures like the Vickers or Knoop indentation tests, which measure absolute hardness by assessing a material’s resistance to indentation under a specific load. These tests provide a more accurate, numerical value for hardness.
In the modern context of materials science, diamond is no longer the hardest material ever discovered. Researchers have successfully synthesized materials that exceed the absolute hardness of natural diamond. Examples include aggregated diamond nanorods (ADNRs) and wurtzite boron nitride (w-BN).
Wurtzite boron nitride, for instance, shares a similar tetrahedral structure with diamond but consists of boron and nitrogen atoms instead of carbon. Although these synthetic superhard materials can be more resistant to indentation or wear in certain conditions, diamond remains the benchmark for the top of the traditional Mohs scale. Diamond is the hardest natural mineral, a title that remains unchallenged.