igneous
Dark glassy stone, possibly obsidian
Obsidian (volcanic glass; chiefly SiO₂), though identification is uncertain from the image
AI-generated identification · may be incorrectSmall dark brown-black, irregular fragment with a dull-to-glassy surface; obsidian has no crystal structure, conchoidal fracture, Mohs hardness about 5–5.5, and specific gravity about 2.3–2.6. The visible piece may instead be another dark volcanic rock or slag.
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Physical properties
Small dark brown-black, irregular fragment with a dull-to-glassy surface; obsidian has no crystal structure, conchoidal fracture, Mohs hardness about 5–5.5, and specific gravity about 2.3–2.6. The visible piece may instead be another dark volcanic rock or slag.
Formation & geological history
Obsidian forms when silica-rich lava cools extremely rapidly, preventing crystal growth, typically during Quaternary volcanic eruptions. Common sources include Mexico, the western United States, Iceland, Italy, and Armenia.
Uses & applications
Used historically for exceptionally sharp tools and arrowheads; attractive varieties are used for carvings, ornaments, cabochons, and teaching specimens. It has little construction value.
Geological facts
Fresh obsidian breaks into razor-sharp edges and may show flow bands, snowflake-like cristobalite inclusions, or a translucent brown sheen. A diagnostic conchoidal fracture is more reliable than color alone.
Field identification & locations
Look for uniform glassiness, sharp curved fractures, and lack of visible grains; a steel file should generally scratch it. The photograph is insufficient for confident identification, and scale, fracture, and a fresh surface would help.
