The question of the hardest natural substance often leads to confusion, as the term “hard” is used differently in everyday conversation than it is in materials science. Many people mistakenly equate hardness with strength, which is a material’s resistance to breaking under a sustained load, or toughness, which is its ability to absorb energy before fracturing. To accurately identify the hardest naturally occurring material, one must understand the specific scientific definition of hardness, which focuses entirely on a material’s surface properties.
Defining Material Hardness
In materials science, hardness is defined as a substance’s resistance to permanent plastic deformation, specifically surface indentation or scratching. It measures how difficult it is to permanently change a material’s shape at a localized point, meaning a hard material strongly resists having its surface abraded. This property differs significantly from material strength, which describes how much force a substance can withstand before it yields or breaks entirely. For example, a ceramic tile is extremely hard and resists scratching, but it has low strength and shatters easily when struck. Toughness measures the energy a material can absorb before fracturing.
Ranking Hardness: The Mohs Scale
The resistance to scratching for natural minerals is most commonly categorized using a system known as the Mohs scale of mineral hardness. Developed in 1812 by German geologist Friedrich Mohs, this qualitative, ordinal system ranks minerals from 1 to 10 based on their ability to scratch one another. A mineral with a higher Mohs number can visibly scratch any mineral with a lower number. The scale begins with Talc at 1, the softest mineral, and progresses through common substances like Quartz at 7.
The Mohs scale is relative and not linear, meaning the difference in absolute hardness between a 5 and a 6 is much smaller than the difference between a 9 and a 10. The scale is valued for its simplicity in field geology, although modern industrial applications require more precise, quantitative scales that measure indentation depth.
Diamond: Structure and Properties
The hardest natural substance on Earth is the diamond, which occupies the highest rank of 10 on the Mohs scale. Its extraordinary resistance to scratching is a direct result of its unique atomic structure. Diamond is an allotrope of carbon, made entirely of carbon atoms, but arranged in a specific crystalline structure.
Every carbon atom within the diamond lattice is bonded to four neighboring carbon atoms in a perfect, repeating tetrahedral arrangement. These connections are formed by incredibly strong covalent bonds that are equal in length and strength throughout the crystal. This rigid, three-dimensional network creates a dense and stable structure that is virtually impossible to distort. The immense energy required to break these uniform covalent bonds grants diamond its unparalleled hardness.
Other Contenders for Hardness
While diamond is the acknowledged leader among natural substances, a few other minerals come close and are often used for comparison. The immediate natural runner-up is corundum, which is ranked at 9 on the Mohs scale and is the mineral group that includes both rubies and sapphires. Corundum is composed of aluminum oxide, and its hardness is a testament to its tight crystal structure, allowing it to scratch all minerals below it, including topaz (8) and quartz (7).
The difference in absolute hardness between corundum and diamond is very large, far exceeding the step between any other two minerals on the Mohs scale. The search for the hardest material continues in the laboratory, where scientists have synthesized materials that exceed the hardness of natural diamond. Cubic Boron Nitride, for example, is a synthetic material that is second only to diamond in hardness, and extremely rare natural substances like Lonsdaleite (a hexagonal form of carbon found only at meteorite impact sites) are theoretically harder than the common cubic diamond.