The question of what constitutes the “most solid thing in the world” cannot be answered with a single material, as the term “solid” is scientifically ambiguous. Materials scientists define a solid body by a combination of distinct physical properties, not a single measure. When people ask this question, they are typically referring to one of three separate material characteristics: resistance to scratching (hardness), ability to withstand force without breaking (strength), or how tightly mass is packed into a given volume (density). The true “most solid” material depends entirely on which of these specific properties is being measured.
Understanding Material Hardness
Hardness is defined as a material’s resistance to permanent localized deformation, such as scratching or indentation. This property relates to the strength and arrangement of atomic bonds within the material’s crystal lattice. The Mohs scale of mineral hardness, which rates materials from 1 (Talc) to 10 (Diamond), is the most common measure for relative scratch resistance.
For precise engineering applications, the Vickers hardness test is used. This test measures the material’s resistance to indentation by a diamond pyramid, yielding a value in Gigapascals (GPa). Natural diamond, a form of carbon with a perfect tetrahedral structure, is the hardest naturally occurring material. It registers a Vickers hardness typically in the 70–150 GPa range, making it the benchmark for scratch resistance.
Scientists have synthesized materials that challenge diamond’s dominance. Cubic Boron Nitride (cBN), with a structure similar to diamond, is the second-hardest bulk material. It is prized for its superior thermal and chemical stability when cutting ferrous metals. Newer synthetic materials, such as Aggregated Diamond Nanorods (ADNRs), are created by compressing fullerene molecules under immense pressure and heat. ADNRs have been reported to exhibit a Vickers hardness up to 150 GPa, making them a strong contender for the hardest synthetic material.
Understanding Material Strength
Material strength describes a material’s ability to withstand an applied stress without fracturing or permanently deforming. This property is more complex than hardness because the failure point depends on the direction and type of force applied. Tensile strength measures the resistance to being pulled apart, while compressive strength measures the resistance to being crushed.
Carbon-based nanomaterials currently hold the record for the highest known tensile strength. Graphene, a single layer of carbon atoms arranged in a honeycomb lattice, possesses a theoretical tensile strength exceeding 130 GPa.
Carbon Nanotubes (CNTs), which are rolled-up sheets of graphene, also exhibit exceptional strength, with individual shells demonstrating tensile strengths up to 100 GPa. This strength results from the strong covalent bonds between carbon atoms and the near-perfect structure of the nanomaterials.
In contrast, the strongest materials against crushing forces are often ceramics or specialized alloys. Diamond holds a record for physical measurement with a compressive strength greater than 110 GPa.
For engineering applications, Bulk Metallic Glasses (BMGs) are noteworthy for their extreme compressive strength and elasticity. These amorphous alloys, such as those based on zirconium, lack the crystalline grain boundaries of traditional metals. This structure allows them to withstand significant crushing forces without yielding.
Understanding Material Density
Density is the measure of mass per unit volume, describing how tightly the atoms or molecules of a material are packed together. This property is independent of a material’s hardness or strength. The densest materials on Earth are found among the metallic elements.
Osmium and Iridium, both noble metals in the platinum group, are the two densest elements under standard conditions. Osmium generally edges out Iridium, possessing a density of approximately 22.59 grams per cubic centimeter. Their high density is due to their large atomic mass combined with a relativistic effect that causes electron orbitals to contract, allowing the atoms to pack more closely.
While Osmium and Iridium are the densest terrestrial materials, they are dwarfed by astronomical objects. A neutron star, the collapsed core of a massive star, represents the theoretical upper limit of density for matter. The material within a neutron star is composed almost entirely of neutrons, reaching densities of up to \(10^{17}\) kilograms per cubic meter. A teaspoon of this material would weigh billions of tons.