igneous
Obsidian
Obsidian (principally SiO2)
Hardness: 5-6 (Mohs scale); Color: Black, sometimes dark brown or green; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.35-2.60.
- Hardness
- 5-6 (Mohs scale)
- Color
- Black, sometimes dark brown or green
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Black, sometimes dark brown or green; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.35-2.60.
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Found in geologically young volcanic areas (Pliocene to Holocene) because it eventually devitrifies into crystals over millions of years.
Uses & applications
Used historically for sharp tools like arrowheads and knives. Modernly used in surgical scalpels, jewelry (cabochons and beads), and as a decorative stone in architecture and interior design.
Geological facts
Because it lack crystals, obsidian can produce edges thinner and sharper than high-quality steel scalpels. It was highly valued in Mayan and Aztec cultures as both a weapon material and a mirror surface.
Field identification & locations
Identify by its glass-like appearance, smooth curved conchoidal fractures, and extremely sharp edges. Commonly found in volcanic regions of Oregon, California, and Iceland. Collectors should look for unique variants like Snowflake or Mahogany Obsidian.
