igneous
Obsidian
Obsidian (Volcanic glass), 70-75% SiO2
Hardness: 5-5.5 (Mohs scale); Color: Black to dark brown with translucent edges; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
- Hardness
- 5-5
- Color
- Black to dark brown with translucent edges
- Luster
- Vitreous/glassy
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black to dark brown with translucent edges; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
Formation & geological history
Formed from extrusive igneous activity where high-silica lava cools extremely rapidly, preventing crystal growth. Most specimens date from the Cenozoic era due to glass devitrification over geological time.
Uses & applications
Used historically for sharp cutting tools (scalpels, arrowheads), modern surgical blades, ornamental jewelry, and holistic healing collections.
Geological facts
Obsidian was prized in the Neolithic era for trading. Because it has no crystalline structure, it can be fractured to produce an edge only molecules thick, which is sharper than high-quality steel.
Field identification & locations
Identify by its distinctive glassy appearance and sharp, curved conchoidal fracture patterns. Found in volcanic regions like the American West, Iceland, and Mexico. Collectors look for unique varieties like Snowflake or Mahogany obsidian.
