igneous
Obsidian (likely black obsidian)
Volcanic glass; predominantly rhyolitic composition (approximately 70–80% SiO₂)
AI-generated identification · may be incorrectUsually black, glassy to waxy, with conchoidal fracture, no visible crystals, and Mohs hardness about 5–5.5; cleavage is absent. The specimen appears rough and freshly fractured, with reflective glass-like surfaces, though identification from the image is not certain.
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Physical properties
Usually black, glassy to waxy, with conchoidal fracture, no visible crystals, and Mohs hardness about 5–5.5; cleavage is absent. The specimen appears rough and freshly fractured, with reflective glass-like surfaces, though identification from the image is not certain.
Formation & geological history
Forms when silica-rich volcanic lava cools so rapidly that crystals cannot grow, commonly during eruptions younger than several million years. It occurs in volcanic domes, lava flows, and marginal zones.
Uses & applications
Used historically for exceptionally sharp blades and arrowheads; today it is collected, carved, and occasionally used in jewelry and decorative objects. It has little construction value compared with ordinary volcanic rock.
Geological facts
Fresh edges can be sharper than surgical steel because fracture produces extremely thin margins. Black obsidian may contain flow bands, gas bubbles, or inclusions that create mahogany, rainbow, or snowflake varieties.
Field identification & locations
Identify by its uniform glassy texture, conchoidal fracture, sharp edges, and lack of grains; unlike basalt, it does not show interlocking crystals. Common localities include Yellowstone, Oregon, Mexico, Armenia, and Italy.
