The question of what constitutes the “hardest object” in the world is complex, as the answer depends entirely on the scientific definition and measurement of hardness. Hardness is not a single, universally defined property but rather a measure of a material’s resistance to permanent change when an external force is applied. Materials science uses different metrics, meaning the hardest substance found in nature and the hardest material created in a laboratory are two distinct answers. This exploration requires understanding how scientists categorize and quantify this extreme property.
Defining Hardness
In materials science, hardness is a property distinct from strength or stiffness, focusing specifically on a material’s resistance to localized deformation. This resistance is characterized by strong chemical bonds between atoms, which resist being broken or rearranged under pressure. Scientists categorize hardness into three principal types to describe different modes of failure under stress.
These types include scratch hardness, which measures resistance to surface abrasion; rebound hardness, which relates to elasticity; and indentation hardness. Indentation hardness is the most relevant metric for identifying the “hardest” materials.
Indentation hardness quantifies a material’s resistance to permanent plastic deformation when a constant compression load is applied by a geometrically defined indenter. For superhard materials, this metric is the most accurate measure of true hardness, as it quantifies resistance to lasting change in shape under extreme pressure. This resistance to plastic flow allows materials to be used for cutting, grinding, and drilling applications.
Quantifying Hardness: The Measurement Scales
The most publicly recognized method of assessing hardness is the Mohs scale of mineral hardness, a qualitative, comparative system that ranks ten minerals based on their ability to scratch one another. While useful for field geology and mineral identification, the Mohs scale is too imprecise for comparing the differences between superhard materials.
To precisely quantify extreme hardness, researchers rely on indentation tests that produce a numerical value, typically expressed in Gigapascals (GPa) or a specific hardness number like HV (Vickers hardness). The Vickers hardness test is one of the most common methods, forcing a square-based pyramidal diamond indenter into the material’s surface under a specific load. The resulting hardness number is calculated from the applied load divided by the surface area of the indentation left behind.
Another precise method is the Knoop hardness test, which uses an elongated pyramidal diamond indenter to create a shallow, rhombic indentation. The Knoop test is useful for measuring the hardness of very small areas, thin films, or brittle materials where a Vickers indenter might cause cracking. Both the Vickers (HV) and Knoop (HK) tests provide the quantifiable data needed to compare the resistance to plastic deformation among the world’s toughest substances.
The Hardest Substance Found in Nature
The natural champion for hardness is diamond, renowned for its unparalleled ability to resist both scratching and indentation. Its extreme hardness results from its unique atomic structure, where each carbon atom is covalently bonded to four neighbors in a perfectly repeating tetrahedral lattice. This arrangement creates a dense, three-dimensional network of the strongest single bonds possible between carbon atoms, requiring immense energy to break or rearrange.
Natural diamonds form deep within the Earth’s mantle under conditions of extremely high pressure and temperature. The resultant cubic crystal structure exhibits a Vickers hardness that typically ranges from 70 to 150 GPa, depending on the crystal orientation and purity.
A less common, naturally occurring variation is Lonsdaleite, sometimes called hexagonal diamond, which is theorized to be even harder than its cubic counterpart. Lonsdaleite is found in trace amounts at meteorite impact sites, where the massive shock of impact transforms graphite into a diamond-like structure. While computational simulations suggest Lonsdaleite could be over 50% harder than conventional diamond, natural specimens are often riddled with structural defects and impurities. This makes their measured hardness highly variable and often lower than perfect cubic diamond.
The Realm of Synthetic Superhard Materials
In modern materials science, the quest for the hardest material has moved beyond natural limits, leading scientists to create synthetic substances that surpass diamond. This research is driven by industrial demand for tools that can cut, drill, and machine increasingly resilient alloys and ceramics. These superhard materials are often produced by modifying the crystal structure of natural compounds or by arranging atoms on a nanoscopic scale.
One such example is Wurtzite Boron Nitride (w-BN), a synthetic material with a structure similar to Lonsdaleite but composed of boron and nitrogen atoms instead of carbon. Data suggest that the unique atomic arrangement in w-BN allows it to resist stress more effectively than diamond, with some measurements indicating an indentation strength around 18% greater than that of cubic diamond. This superior performance is largely attributed to a mechanism where the material’s bonds “flip” under pressure, causing the crystal to rearrange and become stronger.
Aggregated Diamond Nanorods (ADNRs), often referred to as “hyperdiamond,” represent another class of synthetic superhard materials. They are formed by compressing fullerene molecules—spherical carbon cages—under extreme pressure and high temperature. The resulting material is composed of interconnected diamond nanorods, which are more dense and less compressible than regular diamond. ADNRs are scientifically confirmed to be the least compressible material known, effectively taking the title of the hardest substance created in a laboratory.