Igneous
Obsidian
Volcanic glass (SiO2 - Silicon Dioxide)
Hardness: 5-6 (Mohs); Color: Jet black to dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
- Hardness
- 5-6 (Mohs)
- Color
- Jet black to dark brown
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-6 (Mohs); Color: Jet black to dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava. It cools so quickly that mineral crystals do not have time to grow, resulting in a natural glass. Most specimens are geologically young (less than few million years) because glass is unstable and devitrifies over time.
Uses & applications
Used historically for sharp tools like arrowheads and scalpels. Today it is used in decorative jewelry, stone carvings, ornamental pieces, and occasionally high-end surgical blades due to its ability to hold an edge thinner than steel.
Geological facts
Because it is a glass rather than a true mineral crystal, obsidian eventually 'snowflakes' over millions of years as it slowly crystallizes. It was highly prized in pre-Columbian Mesoamerica for both weapons and ritual mirrors.
Field identification & locations
Identify by its distinctive conchoidal (shell-like) fracture patterns and sharp edges. It is often found near volcanic flows in areas like the United States (Oregon, California), Mexico, Iceland, and Japan. Look for a glossy, smooth surface that looks like broken glass.
