igneous
Black obsidian
Volcanic glass; predominantly rhyolitic silica glass (approximately 70–80% SiO₂)
AI-generated identification · may be incorrectTypically black, glossy to resinous, and noncrystalline with conchoidal fracture; Mohs hardness about 5–5.5, no cleavage, and specific gravity around 2.35–2.65. The specimen appears rough, dark, and glassy, though lighting makes exact identification uncertain.
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Physical properties
Typically black, glossy to resinous, and noncrystalline with conchoidal fracture; Mohs hardness about 5–5.5, no cleavage, and specific gravity around 2.35–2.65. The specimen appears rough, dark, and glassy, though lighting makes exact identification uncertain.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, generally during Quaternary to recent volcanic eruptions. Common sources include Mexico, the western United States, Iceland, Armenia, and Japan.
Uses & applications
Used historically for exceptionally sharp blades and arrowheads; today it is used for decorative objects, cabochons, jewelry, and lapidary carvings.
Geological facts
Obsidian is technically a mineraloid rather than a true mineral because it lacks an ordered crystal lattice. Brown, gray, or golden patches may indicate flow banding, inclusions, or weathering.
Field identification & locations
Identify by its glassy surface, very sharp conchoidal edges, lack of visible grains, and inability to scratch ordinary glass reliably; avoid confusing it with black slag or basalt. A definitive check requires examination of fracture and inclusions.
