igneous
Black obsidian
Obsidian, naturally occurring volcanic glass; typically silica-rich rhyolitic composition
AI-generated identification · may be incorrectUsually Mohs 5–5.5; opaque black with a glossy, glassy luster and no crystal structure or cleavage. Breaks with sharp, curved conchoidal fractures; specific gravity is about 2.3–2.6.
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Physical properties
Usually Mohs 5–5.5; opaque black with a glossy, glassy luster and no crystal structure or cleavage. Breaks with sharp, curved conchoidal fractures; specific gravity is about 2.3–2.6.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow. It is geologically young and occurs around volcanic flows and domes.
Uses & applications
Used for ornamental objects, jewelry, and collectible specimens; historically shaped into cutting tools and projectile points.
Geological facts
Its smooth, curved fracture surfaces can be exceptionally sharp. The specimen’s glossy black color and conchoidal breaks are consistent with obsidian.
Field identification & locations
Look for glassy luster, sharp edges, and curved fracture ripples; common sources include volcanic regions such as Mexico, the western United States, and Iceland. Avoid assuming all black volcanic glass is obsidian without checking its texture.
