igneous
Likely obsidian (black volcanic glass)
Obsidian; predominantly rhyolitic volcanic glass, often ~70–75% SiO₂
AI-generated identification · may be incorrectTypically black, dark brown, or gray with vitreous luster, conchoidal fracture, and no visible crystals; Mohs hardness about 5–5.5, specific gravity ~2.3–2.6. The image is blurry, so slag or another dark glassy rock cannot be excluded.
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Physical properties
Typically black, dark brown, or gray with vitreous luster, conchoidal fracture, and no visible crystals; Mohs hardness about 5–5.5, specific gravity ~2.3–2.6. The image is blurry, so slag or another dark glassy rock cannot be excluded.
Formation & geological history
Forms when silica-rich lava cools rapidly, preventing crystallization; most examples are Cenozoic or younger and occur near volcanic domes, flows, and ash deposits.
Uses & applications
Used historically for exceptionally sharp tools and arrowheads; today it is used for ornaments, cabochons, carvings, and collector specimens, with limited decorative aggregate use.
Geological facts
Fresh fractures can be sharper than surgical steel, though obsidian is brittle. Snowflake obsidian contains white cristobalite spherulites.
Field identification & locations
Look for a glassy surface, sharp conchoidal edges, and absence of grain boundaries; common localities include Iceland, Mexico, the western United States, Armenia, and Japan. A streak, hardness, and visual inspection help distinguish it from industrial glass or slag.
