igneous
Tumbled black stone, likely black obsidian
Obsidian (volcanic glass; predominantly SiO₂)
AI-generated identification · may be incorrectDark black-brown, vitreous luster, smooth polished surface, conchoidal fracture, no visible crystals; Mohs hardness about 5–5.5 and specific gravity about 2.3–2.6. The image is insufficient to distinguish obsidian confidently from black tourmaline, onyx, or polished glass.
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Physical properties
Dark black-brown, vitreous luster, smooth polished surface, conchoidal fracture, no visible crystals; Mohs hardness about 5–5.5 and specific gravity about 2.3–2.6. The image is insufficient to distinguish obsidian confidently from black tourmaline, onyx, or polished glass.
Formation & geological history
Obsidian forms when silica-rich lava cools so rapidly that crystals cannot grow, commonly during Quaternary volcanic eruptions. Major sources include Mexico, the western United States, Iceland, Italy, and Armenia.
Uses & applications
Used for ornamental carvings, beads, cabochons, arrowheads, and decorative objects; exceptionally sharp flakes have also been used for cutting tools. Common tumbled pieces have modest collector value.
Geological facts
Obsidian is technically a natural volcanic glass rather than a crystalline mineral. Snowflake, mahogany, rainbow, and sheen varieties arise from inclusions or optical effects.
Field identification & locations
Check for curved conchoidal fracture, glassy luster, and lack of bubbles or mold marks; a steel file should barely scratch it. A loupe, streak, and density test may help separate it from glass or other black minerals.
