igneous
Black obsidian (probable)
Volcanic glass; predominantly SiO₂
AI-generated identification · may be incorrectUsually black, vitreous to glassy luster, conchoidal fracture, no visible crystals or cleavage; Mohs hardness about 5–5.5, specific gravity 2.35–2.70. The image is low-resolution, so slag or another dark volcanic rock cannot be excluded.
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Physical properties
Usually black, vitreous to glassy luster, conchoidal fracture, no visible crystals or cleavage; Mohs hardness about 5–5.5, specific gravity 2.35–2.70. The image is low-resolution, so slag or another dark volcanic rock cannot be excluded.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, generally during Quaternary to older volcanic eruptions. Common around rhyolitic lava flows and volcanic domes.
Uses & applications
Used for ornaments, carvings, cabochons, and historically exceptionally sharp cutting tools; not generally used as structural aggregate. Attractive varieties have modest lapidary value.
Geological facts
Fresh fractures can produce extremely sharp edges, sharper than many steel blades. Snowflake, mahogany, sheen, and rainbow varieties result from crystals, inclusions, or surface effects.
Field identification & locations
Look for a uniformly glassy black surface, curved shell-like fracture, sharp edges, and absence of granular crystals; a streak test is usually unhelpful. Common localities include Mexico, the western United States, Armenia, Turkey, and Iceland.
