igneous
Obsidian
Obsidian (Volcanic Glass, SiO2 + impurities)
Hardness: 5-5.5 (Mohs); Color: Jet black to dark grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (has pronounced conchoidal fracture); Specific Gravity: 2.35–2.6
- Hardness
- 5-5
- Color
- Jet black to dark grey
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs); Color: Jet black to dark grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (has pronounced conchoidal fracture); Specific Gravity: 2.35–2.6
Formation & geological history
Formed through the rapid cooling of silica-rich felsic lava. Because it cools so quickly, crystals do not have time to grow, resulting in a natural glass. It is commonly associated with recent volcanic activity (late Cenozoic era).
Uses & applications
Historically used for arrowheads and cutting tools; currently used in jewelry (beads/cabochons), scalpel blades for high-precision surgery, and decorative ornaments.
Geological facts
Obsidian can be broken to create an edge thinner and sharper than a steel scalpel. Native Americans traded it extensively over thousands of miles. Some varieties like 'Snowflake Obsidian' have white inclusions of cristobalite.
Field identification & locations
Identify by its distinctive shell-like (conchoidal) fracture and sharp edges. It is found in volcanic regions such as the American Northwest (Oregon, California), Iceland, Japan, and Mexico.
