igneous
Obsidian (volcanic glass), likely black obsidian
Rhyolitic volcanic glass; predominantly amorphous SiO₂
AI-generated identification · may be incorrectUsually black to dark brown, vitreous luster, conchoidal fracture, no visible crystals or cleavage; Mohs hardness about 5–5.5, specific gravity 2.35–2.ได้.5. The glossy fractured surfaces and sharp edges are characteristic, though the specimen may be weathered or coated.
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Physical properties
Usually black to dark brown, vitreous luster, conchoidal fracture, no visible crystals or cleavage; Mohs hardness about 5–5.5, specific gravity 2.35–2.ได้.5. The glossy fractured surfaces and sharp edges are characteristic, though the specimen may be weathered or coated.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, typically during late Cenozoic volcanic eruptions. Common in young volcanic fields and rhyolitic lava flows.
Uses & applications
Used for decorative objects, jewelry, cabochons, tools, and historically razor-sharp arrowheads and blades. Value depends mainly on polish, pattern, size, and provenance.
Geological facts
Obsidian is technically a mineraloid rather than a true mineral because it lacks an ordered crystal lattice. Black specimens may contain tiny magnetite crystals or flow bands invisible at low magnification.
Field identification & locations
Identify by its glassy surface, very sharp conchoidal fracture, and lack of grain; it should not fizz in acid and generally scratches glass. Common localities include Mexico, the western United States, Iceland, Armenia, and Japan.
