igneous
Obsidian
Volcanic Glass (Primary composition: SiO2 - Silicon Dioxide)
Hardness: 5-5.5 (Mohs scale); Color: Black to dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Black to dark brown
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black to dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.35-2.6
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, preventing crystal growth. Usually found in geologically young volcanic regions (Neogene to Quaternary) due to its tendency to devitrify over time.
Uses & applications
Used for surgical scalpel blades, ornamental jewelry, decorative carvings, and historically for stone tools (arrowheads, knives). Highly valued by gemstone collectors when displaying 'sheen' or 'snowflake' patterns.
Geological facts
Obsidian is technically a glass, not a mineral, because it lacks a crystalline structure. It fractures with extremely sharp edges that can be much thinner and sharper than a steel scalpel.
Field identification & locations
Identify in the field by its smooth, glassy texture and characteristic curved (conchoidal) fracture lines. Common in the Western US (Oregon, California), Iceland, and Mexico. Collectors should look for unique inclusions or translucency on thin edges.
