igneous
Obsidian
Volcanic Glass (Primary component: SiO2)
Hardness: 5-5.5 (Mohs scale); Color: Typically black but can be green, brown, or mottled; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (Conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- Typically black but can be green, brown, or mottled
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Typically black but can be green, brown, or mottled; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (Conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Formed from extrusive igneous activity when silica-rich lava cools too rapidly for crystals to grow. Most common in the Cenozoic era due to its tendency to devitrify over geological time.
Uses & applications
Historically used for edged tools (scalpels, arrowheads). Modern uses include jewelry (cabochons), ornamental masks, and specialized surgical blades due to its molecular sharpness.
Geological facts
Obsidian fractures with an edge thinner than the sharpest steel scalpel. 'Snowflake Obsidian' forms when crystals of cristobalite begin to grow within the glass, creating white flower-like patterns.
Field identification & locations
Identify by its distinctive conchoidal (shell-like) fracture patterns and sharp edges. Found primarily in high-silica volcanic regions. Key locations include the Western United States (Oregon, California), Iceland, and Mexico.
