igneous
Obsidian (likely black volcanic glass)
Obsidian; predominantly silica-rich rhyolitic volcanic glass (approximately 70– on 80% SiO₂)
AI-generated identification · may be incorrectUsually black, glassy, and opaque to translucent at thin edges, with conchoidal fracture and no visible crystals; Mohs hardness about 5–5.5, vitreous luster, and specific gravity around 2.3–2.6. The rough, sharp-edged fragment is consistent with fractured obsidian, though the photo alone cannot exclude another dark gla…
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Physical properties
Usually black, glassy, and opaque to translucent at thin edges, with conchoidal fracture and no visible crystals; Mohs hardness about 5–5.5, vitreous luster, and specific gravity around 2.3–2.6. The rough, sharp-edged fragment is consistent with fractured obsidian, though the photo alone cannot exclude another dark glassy rock.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, commonly during Quaternary to younger volcanic eruptions. It occurs in rhyolitic lava flows, domes, and volcanic ash environments.
Uses & applications
Used historically for exceptionally sharp tools and arrowheads; today it is collected, carved, polished, and occasionally used for decorative objects and jewelry.
Geological facts
Obsidian is technically a mineraloid rather than a true mineral because it lacks an ordered crystal structure. Flow banding, snowflake patterns, or translucent brown/gray coloration can occur.
Field identification & locations
Identify it by glassy luster, very sharp conchoidal edges, lack of grain, and inability to scratch ordinary glass reliably; common sources include Iceland, Mexico, the western United States, Armenia, and Japan.
