igneous
Black Obsidian
Obsidian (Volcanic glass), 70–75% SiO2
Hardness: 5–5.5 Mohs; Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.35–2.6
- Hardness
- 5–5
- Color
- Jet black
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5–5.5 Mohs; Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.35–2.6
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is found in geologically recent volcanic areas (Cenozoic era).
Uses & applications
Used for surgical scalpel blades, ornamental jewelry, decorative carvings, and historically for arrowheads and cutting tools.
Geological facts
Because it has no crystal structure, obsidian can be fractured to create an edge thinner than a steel scalpel, reaching molecular thinness. It was highly prized by civilizations like the Aztecs.
Field identification & locations
Identify by its glass-like texture and curved, shell-like (conchoidal) fracture patterns. Found in volcanic regions like the American West (Iceland, Mexico, Japan). Collectors look for high translucency on edges.
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