igneous
Obsidian
Volcanic Glass (SiO2 + impurities)
Hardness: 5–5.5 on Mohs scale; Color: typically jet black, can be dark green or brown; Luster: vitreous (glassy); Crystal structure: amorphous (none); Cleavage: none (conchoidal fracture); Specific gravity: 2.35–2.6.
- Hardness
- 5–5
- Color
- typically jet black, can be dark green or brown
- Luster
- vitreous (glassy)
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Physical properties
Hardness: 5–5.5 on Mohs scale; Color: typically jet black, can be dark green or brown; Luster: vitreous (glassy); Crystal structure: amorphous (none); Cleavage: none (conchoidal fracture); Specific gravity: 2.35–2.6.
Formation & geological history
Formed from extrusive igneous activity when highly viscous lava (rich in silica) cools extremely rapidly, preventing crystal growth. Usually found in geologically young volcanic areas (Cenozoic to Holocene).
Uses & applications
Used historically for sharp stone tools and weapons (arrowheads, scalpels). Modern uses include jewelry, decorative sculptures, and high-precision surgical scalpel blades.
Geological facts
Because it is glass, obsidian breaks with very sharp edges (conchoidal fracture). It is metastable and over millions of years will eventually devitrify into fine-grained quartz crystals, which is why ancient obsidian from the Paleozoic era is rare.
Field identification & locations
Identify by its incredible glass-like appearance, lack of grain, and sharp, curved fracture patterns. Commonly found near volcanic sites like the Cascades in the US, Iceland, and Mexico.
