igneous
Polished black stone, likely black obsidian
Obsidian (volcanic glass), primarily silica-rich rhyolitic composition
AI-generated identification · may be incorrectTypically black to dark brown, vitreous luster, smooth conchoidal surfaces, and no visible crystals; hardness about 5–5.5 Mohs, amorphous structure, no cleavage, and specific gravity around 2.3–2.6. The image shows a small rounded, highly polished specimen, though exact identification cannot be confirmed visually.
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Physical properties
Typically black to dark brown, vitreous luster, smooth conchoidal surfaces, and no visible crystals; hardness about 5–5.5 Mohs, amorphous structure, no cleavage, and specific gravity around 2.3–2.6. The image shows a small rounded, highly polished specimen, though exact identification cannot be confirmed visually.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, generally during Quaternary to younger volcanic eruptions. Commonly associated with rhyolite domes and lava flows.
Uses & applications
Used for ornamental objects, cabochons, beads, carvings, and historically sharp tools and arrowheads. Small polished pieces generally have modest decorative value.
Geological facts
Obsidian can fracture into exceptionally sharp edges and may display sheen, snowflake patterns, or rainbow colors from microscopic inclusions. It is technically volcanic glass rather than a crystalline mineral.
Field identification & locations
Look for glassy luster, uniform dark color, conchoidal fracture, and lack of grain; it should not fizz in acid and usually will not scratch quartz. Common sources include Mexico, the western United States, Armenia, Iceland, and Japan.
