igneous
Tumbled black obsidian
Obsidian, a natural volcanic glass composed chiefly of silica-rich, felsic melt
AI-generated identification · may be incorrectBlack, glassy luster and conchoidal fractures before tumbling; Mohs hardness about 5–5.5. It is amorphous, with no crystal structure or cleavage.
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Physical properties
Black, glassy luster and conchoidal fractures before tumbling; Mohs hardness about 5–5.5. It is amorphous, with no crystal structure or cleavage.
Formation & geological history
Forms when silica-rich lava cools rapidly, preventing crystals from growing; specimens occur in volcanic regions of many ages, commonly Quaternary.
Uses & applications
Used for lapidary jewelry, ornaments, and historically for very sharp cutting tools.
Geological facts
Obsidian is volcanic glass rather than a crystalline mineral; its smooth, curved fractures can produce exceptionally sharp edges.
Field identification & locations
The uniformly dark, glossy appearance and glass-like fracture suggest obsidian, though black onyx or another polished stone can look similar. Common sources include Mexico, the western United States, Iceland, and Japan.
