Obsidian is a naturally occurring volcanic glass, formed when lava extruded from a volcano cools rapidly. This rapid cooling prevents the formation of crystalline structures, resulting in a glassy, amorphous texture. This texture is a significant factor in determining its physical properties. The standard method for assessing this property is the Mohs scale of mineral hardness.
Understanding the Mohs Scale
The Mohs scale provides a simple, qualitative measure of a material’s resistance to scratching by comparing it against ten reference minerals. Developed in 1812 by German mineralogist Friedrich Mohs, the scale ranges from 1 (Talc, the softest) to 10 (Diamond, the hardest).
This system is an ordinal scale, meaning it ranks materials in order but does not represent a linear increase in hardness. For example, the difference in hardness between Diamond (10) and Corundum (9) is much greater than the difference between any other two consecutive minerals. The Mohs test determines hardness by finding the hardest material that a specimen can scratch, and the softest material that can scratch the specimen.
Obsidian’s Specific Hardness Rating
Obsidian registers a hardness rating between 5 and 6 on the Mohs scale. Because it is an amorphous volcanic glass, the most commonly cited range is 5 to 5.5. This moderate scratch resistance means obsidian is harder than materials like Apatite (5) but softer than Orthoclase Feldspar (6).
This rating is comparable to common materials encountered in daily life. Standard window glass has a Mohs hardness of approximately 5.5, and a steel knife blade typically falls in the 5.5 to 6.5 range. Variation in obsidian’s rating is often due to the presence of minor impurities or differences in the material’s internal porosity.
Practical Implications of Obsidian’s Hardness
Obsidian’s moderate Mohs hardness is viewed alongside its unique fracture pattern to explain its utility. While a hardness of 5.5 means it is susceptible to scratching by harder minerals like Quartz (7), its glassy structure lacks the internal planes of weakness found in crystalline materials. This absence of crystalline structure leads to conchoidal fracture.
Conchoidal fracture, named for its shell-like, smoothly curved breakage surface, is the source of obsidian’s extreme sharpness. When struck, the homogeneous volcanic glass breaks, creating edges tapering to a microscopic fineness. This characteristic allowed prehistoric cultures to shape it into exceptionally sharp tools and arrowheads through a process called knapping.
The resulting edge can be many times sharper than a steel surgical scalpel. A well-crafted obsidian blade can have a cutting edge as thin as three nanometers. This ultra-fine edge causes less trauma to tissue, leading to its specialized use in some delicate surgical procedures. The material’s relative softness, which makes it vulnerable to scratching, is what allows it to be brittle and break into sharp, thin flakes.