igneous
Polished black stone, likely obsidian
Volcanic glass (primarily SiO₂), probably black obsidian
AI-generated identification · may be incorrectUsually black with a vitreous, glassy luster; amorphous rather than crystalline, with no cleavage. Mohs hardness is about 5–5.5; the rounded or polished surface obscures its characteristic conchoidal fracture.
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Physical properties
Usually black with a vitreous, glassy luster; amorphous rather than crystalline, with no cleavage. Mohs hardness is about 5–5.5; the rounded or polished surface obscures its characteristic conchoidal fracture.
Formation & geological history
Obsidian forms when silica-rich lava cools so rapidly that crystals cannot grow, commonly during volcanic eruptions in the Quaternary and older geologic periods. Exact identification is limited by the small, highly polished specimen and lighting.
Uses & applications
Used historically for sharp tools and arrowheads, and today for ornamental objects, cabochons, beads, and decorative carving. Common specimens generally have modest collector value.
Geological facts
The apparent pale patches may be reflections, inclusions, or flow-related material; snowflake obsidian would contain recognizable white cristobalite spherulites. Black obsidian can resemble jet or industrial glass.
Field identification & locations
Common localities include Mexico, the western United States, Iceland, Armenia, and Japan. Look for a lightweight glassy feel, sharp conchoidal edges on broken surfaces, and absence of visible mineral grains; a streak and refractive appearance help distinguish it from crystalline rocks.
