igneous
Black obsidian with rhyolitic inclusions
Obsidian, volcanic glass; predominantly silica-rich rhyolitic composition (approximately 70–75% SiO₂)
AI-generated identification · may be incorrectTypically black, vitreous, and highly reflective with conchoidal fracture; Mohs hardness about 5–5.5, no cleavage, and specific gravity around 2.35–2.65. The pale tan crystals or patches are likely feldspar and/or quartz inclusions or weathered matrix.
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Physical properties
Typically black, vitreous, and highly reflective with conchoidal fracture; Mohs hardness about 5–5.5, no cleavage, and specific gravity around 2.35–2.65. The pale tan crystals or patches are likely feldspar and/or quartz inclusions or weathered matrix.
Formation & geological history
Forms when silica-rich lava cools rapidly, preventing crystal growth, generally in Quaternary to younger volcanic environments. The rough outer material may represent partially crystallized rhyolite or perlitic/weathered margins.
Uses & applications
Used for ornamental objects, carvings, cabochons, arrowheads, and collector specimens; historically valued for producing exceptionally sharp cutting edges. It has limited construction use compared with crystalline volcanic rocks.
Geological facts
Its glassy texture reflects rapid cooling rather than melting alone. Fresh broken surfaces can be razor sharp, and flow bands or small trapped crystals are common.
Field identification & locations
Identify by its glassy luster, curved conchoidal fractures, lack of visible overall grain, and sharp edges; common localities include Mexico, the western United States, Armenia, Italy, and Japan. Estimated value: $5–$30 for a common rough specimen, or $30–$100+ for attractive inclusions and display quality.
