igneous
Obsidian (likely black volcanic glass)
Obsidian; mostly amorphous SiO₂-rich rhyolitic glass
AI-generated identification · may be incorrectUsually black, vitreous to pitchy luster, conchoidal fracture, no visible crystals or cleavage; Mohs hardness about 5–5.5, specific gravity 2.35–2.65. The glossy, sharp-edged appearance is consistent with obsidian, though lighting obscures details.
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Physical properties
Usually black, vitreous to pitchy luster, conchoidal fracture, no visible crystals or cleavage; Mohs hardness about 5–5.5, specific gravity 2.35–2.65. The glossy, sharp-edged appearance is consistent with obsidian, though lighting obscures details.
Formation & geological history
Forms when silica-rich volcanic lava cools so rapidly that crystals cannot grow, generally during relatively young volcanic eruptions from the Cenozoic to present. Common around rhyolitic lava domes and flows.
Uses & applications
Used for ornamental objects, cutting tools, arrowheads, cabochons, and polished specimens; high-quality varieties are used in jewelry and sculpture.
Geological facts
Fresh fracture surfaces can be razor sharp. Black obsidian may contain microscopic crystals, gas bubbles, or flow banding that produce sheen, snowflake, or rainbow varieties.
Field identification & locations
Common localities include the western United States, Mexico, Iceland, Armenia, Turkey, and Japan. Confirm with conchoidal fracture and glassy texture; do not confuse it with industrial glass or coal.
