igneous
Obsidian (volcanic glass)
Obsidian; predominantly rhyolitic volcanic glass, mainly SiO₂ with minor feldspar, quartz, and iron oxides
AI-generated identification · may be incorrectUsually black with a vitreous to waxy luster, conchoidal fracture, and no visible crystals; Mohs hardness about 5–5.5, non-cleavable, specific gravity roughly 2.3–2.6. The sharp curved edges and glassy surfaces strongly fit obsidian, though the porous texture may indicate vesicular or weathered material.
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Physical properties
Usually black with a vitreous to waxy luster, conchoidal fracture, and no visible crystals; Mohs hardness about 5–5.5, non-cleavable, specific gravity roughly 2.3–2.6. The sharp curved edges and glassy surfaces strongly fit obsidian, though the porous texture may indicate vesicular or weathered material.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, commonly during Quaternary volcanic eruptions. It occurs around rhyolitic lava domes and flows, often alongside pumice and perlite.
Uses & applications
Used historically for razor-sharp tools and arrowheads; today it is collected, carved, polished, and occasionally used in jewelry and decorative objects.
Geological facts
Fresh obsidian can produce edges sharper than surgical steel, but it is brittle and easily chipped. Black, mahogany, snowflake, rainbow, and sheen varieties are recognized.
Field identification & locations
Identify by conchoidal fracture, glassy sheen, light weight, and lack of grain; it commonly occurs in volcanic regions such as the western United States, Mexico, Iceland, and Armenia. The specimen appears likely obsidian, but a photo cannot exclude industrial glass or slag.
