The question of whether obsidian is harder than diamond has a clear answer: no, obsidian is not harder. Diamond is the hardest naturally occurring substance known on Earth, while obsidian is a form of volcanic glass. Diamond is a pure element with a highly ordered crystalline structure. Obsidian, conversely, is a silica-rich compound lacking that rigid atomic arrangement. This difference in fundamental composition and internal structure makes diamond vastly superior in hardness.
What Does ‘Hardness’ Mean?
In materials science, “hardness” is defined as a substance’s resistance to localized plastic deformation, such as scratching or abrasion. The Mohs scale of mineral hardness is the most common system for comparing this property. This qualitative scale, ranging from 1 to 10, ranks materials based on their ability to scratch one another. A substance with a higher Mohs number can scratch any material with a lower number. Hardness is distinct from toughness, which is a material’s resistance to fracturing or breaking when subjected to impact.
Diamond’s Atomic Structure and Strength
Diamond sits at the top of the Mohs scale, scoring 10, which indicates the greatest resistance to scratching of any natural material. Its extreme hardness stems from its unique atomic structure. Diamond is composed solely of carbon atoms, each joined to four others in a repeating, three-dimensional geometric pattern. This arrangement forms a dense and highly stable crystal lattice known as a diamond cubic structure. The strong covalent bonds holding these atoms together are exceptionally difficult to break, giving diamond unparalleled abrasive resistance.
Obsidian’s Composition and Relative Softness
Obsidian is a naturally occurring igneous rock that forms when felsic lava cools so rapidly that mineral crystals do not have time to grow. It is essentially a volcanic glass, composed of about 65 to 80 percent silica (\(\text{SiO}_2\)) and other lighter elements. Because the atoms in obsidian did not arrange into an ordered, repeating crystal lattice, its structure is considered amorphous, or lacking a defined shape. This structural difference results in obsidian being significantly softer than diamond, typically rating between 5 and 6 on the Mohs scale. Due to its glassy nature, obsidian fractures easily with a characteristic conchoidal fracture, producing extremely sharp edges, but this characteristic is a measure of its brittleness, not its hardness.