igneous
Polished black stone, likely black obsidian
Obsidian — volcanic glass, chiefly SiO₂
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; opaque black with a vitreous, highly reflective luster and conchoidal fracture. It lacks a true crystal structure, cleavage, and visible grains; the smooth shape is human-polished.
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Physical properties
Hardness about 5–5.5 Mohs; opaque black with a vitreous, highly reflective luster and conchoidal fracture. It lacks a true crystal structure, cleavage, and visible grains; the smooth shape is human-polished.
Formation & geological history
Rapid cooling of silica-rich lava prevented crystal growth, commonly during rhyolitic eruptions in the Quaternary and older volcanic fields. The image alone cannot distinguish it confidently from black jasper, basalt, or manufactured glass.
Uses & applications
Used for cabochons, beads, carvings, ornaments, and historically razor-sharp tools and arrowheads; generally unsuitable for structural construction.
Geological facts
Fresh obsidian breaks into exceptionally sharp edges and may show flow bands, gray sheen, or “snowflake” cristobalite inclusions. It is commonly associated with rhyolite and volcanic domes.
Field identification & locations
Likely obsidian if its broken edge is glassy and conchoidal, it has no bubbles, and a steel point can scratch it only with difficulty; origin and testing are needed for certainty. Common sources include Mexico, the western United States, Armenia, Italy, and Japan.
