igneous
Obsidian
Volcanic Glass (SiO2)
Hardness: 5–5.5 Mohs; Color: Jet black (can be dark green or brown); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.3–2.6
- Hardness
- 5–5
- Color
- Jet black (can be dark green or brown)
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5–5.5 Mohs; Color: Jet black (can be dark green or brown); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.3–2.6
Formation & geological history
Formed through the rapid cooling of silica-rich felsic lava, which prevents crystal growth. It occurs in geologically recent volcanic areas where there has been extrusive cooling.
Uses & applications
Used in gemstone jewelry, ornamental carvings, and historical blades. Modern medical use includes high-precision scalpel blades. Highly valued by collectors for tumble-polishing.
Geological facts
Obsidian is fragile and breaks into pieces with curved surfaces that are thinner and sharper than high-quality steel. It was used in pre-Columbian Mesoamerica for weapons and tools because of this sharpness.
Field identification & locations
Identify by its glassy texture, shell-like (conchoidal) fracture pattern, and intense black color. Commonly found in volcanic regions like the American Northwest (Oregon, California), Mexico, and Iceland.
