igneous
Obsidian
Obsidian (Volcanic Glass, SiO2 + impurity)
Hardness: 5-5.5 (Mohs scale); Color: Black (usually); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
- Hardness
- 5-5
- Color
- Black (usually)
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black (usually); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
Formation & geological history
Formed through the rapid cooling of silica-rich felsic lava, which prevents the growth of crystals. Typically found in geologically recent volcanic areas like the Cascades or the Andes.
Uses & applications
Used in ornamental jewelry, decorative items, and historically for surgical scalpels and ancient weaponry (arrowheads, blades).
Geological facts
Obsidian is technically a glass, not a mineral, because it lacks a crystalline structure. Ancient Aztecs used obsidian mirrors for divination.
Field identification & locations
Identify by its sharp, curved (conchoidal) edges and extremely smooth, glassy surface. Commonly found near rhyolitic volcanic flows in the Western US, Mexico, and Iceland.
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