igneous
Obsidian
Obsidian (volcanic glass)
Hardness: 5-6 (Mohs); Color: Black, deep green, or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.35-2.6
- Hardness
- 5-6 (Mohs)
- Color
- Black, deep green, or brown
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-6 (Mohs); Color: Black, deep green, or brown; 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. Found in regions with recent volcanic activity (late Cenozoic-Quaternary).
Uses & applications
Used historically for sharp tools like arrowheads and scalpels. In modern times, used for jewelry, gemstone carving, ornamental decorative objects, and collectible specimens.
Geological facts
Because it is glass and not a crystal, obsidian is chemically unstable over long geological periods and eventually devitrifies into quartz or feldspar. It was highly valued by ancient civilizations for its razor-sharp edges.
Field identification & locations
Identify by its extremely smooth, glassy texture and conchoidal (shell-like) fracture patterns. Common in volcanic areas like the American West, Mexico, and Iceland. Collectors look for unique varieties like snowflake or rainbow obsidian.
