mineral
Black glassy fragment, likely obsidian or industrial glass
Volcanic glass (obsidian), chiefly amorphous SiO₂; identification uncertain from image
AI-generated identification · may be incorrectBlack, highly vitreous luster with sharp conchoidal-looking fractures; obsidian has Mohs hardness about 5–5.5, no cleavage, and low-to-moderate specific gravity (~2.3–2.6). The surrounding surface appears concrete or asphalt, but the dark fragment may be natural obsidian or man-made glass.
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Physical properties
Black, highly vitreous luster with sharp conchoidal-looking fractures; obsidian has Mohs hardness about 5–5.5, no cleavage, and low-to-moderate specific gravity (~2.3–2.6). The surrounding surface appears concrete or asphalt, but the dark fragment may be natural obsidian or man-made glass.
Formation & geological history
Obsidian forms when silica-rich lava cools rapidly, preventing crystal growth, typically during Quaternary volcanic activity; industrial glass forms by melting and cooling manufactured silica-rich material.
Uses & applications
Obsidian is used for ornamental objects, tools, and collectible specimens; industrial glass is used in construction and manufacturing. This small fragment has little commercial value.
Geological facts
Obsidian can produce exceptionally sharp edges and was widely used for prehistoric blades and arrowheads. Black glass alone is not diagnostic because slag and broken bottle glass can look similar.
Field identification & locations
Check for flow banding, tiny mineral inclusions, and uniformly conchoidal fracture for obsidian; bubbles, straight manufactured surfaces, or embedded debris suggest glass or slag. Common obsidian sources include the western United States, Mexico, Iceland, and Japan.
