igneous
Obsidian
Volcanic Glass (Amorphous SiO2)
Hardness: 5 to 5.5 on the Mohs scale. Color: Deep black, sometimes brown, tan, or with a sheen. Luster: Vitreous (glassy). Crystal structure: Amorphous (non-crystalline, volcanic glass). Cleavage: None (exhibits distinct conchoidal fracture). Specific gravity: 2.35 - 2.60.
- Hardness
- 5 to 5
- Color
- Deep black, sometimes brown, tan, or with a sheen
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5 to 5.5 on the Mohs scale. Color: Deep black, sometimes brown, tan, or with a sheen. Luster: Vitreous (glassy). Crystal structure: Amorphous (non-crystalline, volcanic glass). Cleavage: None (exhibits distinct conchoidal fracture). Specific gravity: 2.35 - 2.60.
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimum crystal growth. It typically forms within the margins of rhyolitic lava flows.
Uses & applications
Historically used for making cutting tools, arrowheads, and spear points due to its exceptionally sharp edges; currently used for decorative objects, jewelry, and occasionally in surgical scalpel blades.
Geological facts
Because obsidian cools so quickly, it has no crystal structure and is technically a mineraloid rather than a true mineral. Freshly broken obsidian has a cutting edge sharper than high-quality surgical steel.
Field identification & locations
Easily identified by its jet-black color, glass-like luster, and sharp conchoidal fracture lines (visible as circular ripples on the surface). Found worldwide in areas with past volcanic activity, such as the western United States, Mexico, Italy, and Iceland.
