igneous
Obsidian
Volcanic Glass (SiO2 + minor impurities)
Hardness: 5-5.5 (Mohs); Color: jet black, dark gray/brown; Luster: vitreous (glassy); Crystal structure: amorphous (none); Cleavage: none, shows conchoidal fracture; Specific gravity: 2.35-2.6.
- Hardness
- 5-5
- Color
- jet black, dark gray/brown
- Luster
- vitreous (glassy)
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Physical properties
Hardness: 5-5.5 (Mohs); Color: jet black, dark gray/brown; Luster: vitreous (glassy); Crystal structure: amorphous (none); Cleavage: none, shows conchoidal fracture; Specific gravity: 2.35-2.6.
Formation & geological history
Formed from extrusive volcanic active; specifically when highly viscous, silica-rich lava cools extremely rapidly, preventing internal crystal growth. Found in geologically recent volcanic fields.
Uses & applications
Used in surgical scalpel blades for precision, jewelry (pendants, beads), landscaping, and high-end mirrors. Historically used for arrowheads and scrapers.
Geological facts
Obsidain is technically a glass rather than a mineral because it lacks a crystalline structure. Ancient Aztecs used obsidian mirrors for divination, associating it with the god Tezcatlipoca.
Field identification & locations
Identify by its glass-like texture and sharp, curved (conchoidal) edges when broken. Commonly found in locations like Glass Buttes (Oregon), Yellowstone, or volcanic regions in Mexico and Iceland.
