igneous
Obsidian
Volcanic Glass (Amorphous Silicon Dioxide, SiO2)
Hardness: 5–6 Mohs; Color: Jet black to dark green; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.35–2.6.
- Hardness
- 5–6 Mohs
- Color
- Jet black to dark green
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5–6 Mohs; Color: Jet black to dark green; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.35–2.6.
Formation & geological history
Formed from extrusive igneous activity when felsic lava cools rapidly with minimal crystal growth. Found in geologically young volcanic areas (Quaternary age) because glass eventually devitrifies into crystalline rock.
Uses & applications
Historically used for arrowheads and cutting tools; currently used in jewelry (cabochons), as reflective mirrors in decorative art, and in specialized surgical scalpel blades.
Geological facts
Obsidian fractures with extremely sharp edges—thinner than steel—making it useful for microsurgery. Famous examples include the 'Apache Tears' found in the American Southwest.
Field identification & locations
Identify by its glass-like texture, sharp edges, and unique curved (conchoidal) fracture pattern. Commonly found in Oregon, Mexico, and Iceland near volcanic domes. Collectors look for high translucency or 'sheen'.
