igneous
Obsidian (volcanic glass)
Obsidian, naturally occurring rhyolitic volcanic glass; composition is silica-rich, commonly about 70–75% SiO₂
AI-generated identification · may be incorrectBlack, shiny, with a glassy luster and curved conchoidal fractures; no visible crystals or cleavage. Hardness is about 5–5.5 Mohs; specific gravity is typically 2.3–2.6.
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Physical properties
Black, shiny, with a glassy luster and curved conchoidal fractures; no visible crystals or cleavage. Hardness is about 5–5.5 Mohs; specific gravity is typically 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 centers and lava flows.
Uses & applications
Historically used for sharp tools and blades; today it is collected, carved, and occasionally made into jewelry. It has limited use as a gemstone because it can chip easily.
Geological facts
Obsidian is natural glass, not a mineral, because it lacks an ordered crystal structure. Its sharp fracture edges have been used for cutting tools since prehistoric times.
Field identification & locations
Identify by its glassy black surface and shell-like, sharp-edged fractures; common sources include Iceland, Mexico, the western United States, and Japan. The pictured specimen appears consistent with rough obsidian.
