Obsidian is a volcanic glass, so its strength depends on what property you mean, but in practical terms it is relatively soft and brittle compared with many common hard rocks and metals, yet it can take a very sharp edge. Key points

• Hardness: Obsidian sits around 5 on the Mohs scale, roughly the same as glass, and softer than minerals like quartz. This means it resists scratching less than harder minerals but can still be fairly resistant to everyday wear compared with softer rocks.

• Brittleness and fracture: It is very brittle and tends to fracture with conchoidal (curved) fractures when struck, producing sharp edges. This brittleness is what makes it excellent for sharp tools but also means it can shatter if dropped or struck hard.

• Edge quality: When fractured, obsidian forms extremely sharp edges, historically exploited for blades and arrowheads. This high edge sharpness comes from its amorphous structure (lacking crystals).

• Compressive strength and practical toughness: Obsidian generally has relatively low compressive strength compared with crystalline rocks and metals, contributing to its brittleness, though exact values vary by sample and measurement. In general, it is strong for forming a very fine edge but not for withstanding heavy impacts or sustained bending.

• Density and composition: It is silica-rich (SiO2) with densities around 2.55–2.70 g/cm³, typical for glassy rocks, which relates to its mechanical behavior but not to “strength” in the sense of load-bearing materials.

Practical takeaways

• For blades and cutting tools, obsidian can be superior to many other local stones in edge sharpness, but it is extremely brittle and can fracture under impact. Proper use protects against chipping or shattering.
• For structural or load-bearing applications, obsidian is not suitable due to its brittleness and relatively low overall toughness compared with crystalline rocks or metals.

If you have a specific context (e.g., blade-making, armor, or a geology classroom comparison), I can tailor the details to that scenario and cite more precise figures.