igneous
Obsidian (likely black obsidian)
Volcanic glass; chiefly silica-rich rhyolitic composition, approximately SiO₂ 70–75%
AI-generated identification · may be incorrectTypically black to dark brown with vitreous luster, conchoidal fracture, and no visible crystals; Mohs hardness about 5–5.5, specific gravity 2.3–2.6, and no cleavage. The sharp curved edges and glassy surface are characteristic, though the specimen appears weathered or partly altered.
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Physical properties
Typically black to dark brown with vitreous luster, conchoidal fracture, and no visible crystals; Mohs hardness about 5–5.5, specific gravity 2.3–2.6, and no cleavage. The sharp curved edges and glassy surface are characteristic, though the specimen appears weathered or partly altered.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, usually in volcanic flows or domes. Most examples are Cenozoic or younger, although obsidian occurs in volcanic terrains of many ages.
Uses & applications
Used historically for cutting tools and arrowheads because it fractures into extremely sharp edges; today it is collected, carved, and occasionally used for jewelry or decorative objects.
Geological facts
Obsidian is technically a natural glass rather than a crystalline mineral. Hydration dating can estimate the age of freshly exposed surfaces, and varieties include snowflake, mahogany, rainbow, and sheen obsidian.
Field identification & locations
Identify it by its glassy luster, conchoidal fracture, lack of grains, and sharp edges; unlike slag, it commonly lacks bubbles and has natural flow banding. Common localities include Mexico, the western United States, Armenia, Iceland, Italy, and Japan.
