igneous
Likely obsidian
Volcanic glass, predominantly silicic SiO₂
AI-generated identification · may be incorrectBlack, glassy to dull luster with conchoidal fracture and no visible crystals; Mohs hardness about 5–5.5, nonporous, and typically lacking cleavage. The irregular dark-gray piece may also be basalt or slag, but its sharp glass-like fracture favors obsidian.
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Physical properties
Black, glassy to dull luster with conchoidal fracture and no visible crystals; Mohs hardness about 5–5.5, nonporous, and typically lacking cleavage. The irregular dark-gray piece may also be basalt or slag, but its sharp glass-like fracture favors obsidian.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, generally during Quaternary volcanic eruptions, though older deposits are common. Typical sources include volcanic fields in the western United States, Mexico, Iceland, Armenia, and Japan.
Uses & applications
Used historically for exceptionally sharp cutting tools and arrowheads; today it is collected, carved, and occasionally used for jewelry or ornamental objects. It has little construction value.
Geological facts
Obsidian edges can be sharper than surgical steel, but the material is brittle and flakes easily. Snowflake, mahogany, rainbow, and sheen varieties result from inclusions or optical effects.
Field identification & locations
Identify by its uniform glassy appearance, curved shell-like fracture, sharp edges, and lack of granular crystals; a steel file usually scratches it only weakly. Common field localities include perlitic lava flows and volcanic domes.
