igneous
Likely obsidian fragment
Volcanic glass; predominantly SiO₂ (amorphous, rhyolitic composition)
AI-generated identification · may be incorrectDark gray to black, vitreous luster, conchoidal fracture, no visible crystals or cleavage; Mohs hardness about 5–5.5, specific gravity 2.3–2.6. The small triangular piece appears naturally fractured or cut, though the image is not diagnostic.
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Physical properties
Dark gray to black, vitreous luster, conchoidal fracture, no visible crystals or cleavage; Mohs hardness about 5–5.5, specific gravity 2.3–2.6. The small triangular piece appears naturally fractured or cut, though the image is not diagnostic.
Formation & geological history
Forms when silica-rich lava cools rapidly, preventing crystal growth, typically during Quaternary to older volcanic eruptions. Common in rhyolitic volcanic fields and lava domes.
Uses & applications
Used historically for exceptionally sharp blades and arrowheads; today it is collected, carved, and occasionally used in jewelry or decorative objects.
Geological facts
Obsidian edges can be sharper than surgical steel, but the material is brittle and produces razor-sharp flakes.
Field identification & locations
Identify by its glassy appearance, curved shell-like fracture, and lack of crystals; test cautiously because edges may cut. Common localities include Arizona, California, Oregon, Mexico, Iceland, and Japan.
