igneous
Polished black stone, likely black obsidian
Obsidian (volcanic glass), predominantly rhyolitic silica glass
AI-generated identification · may be incorrectThe specimen is a smooth, glossy, opaque-to-translucent black cabochon with conchoidal fracture. Obsidian has Mohs hardness about 5–5.5, vitreous luster, no cleavage, and no crystal structure; exact identification is uncertain from the image alone because black agate, onyx, or tourmaline are possible.
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Physical properties
The specimen is a smooth, glossy, opaque-to-translucent black cabochon with conchoidal fracture. Obsidian has Mohs hardness about 5–5.5, vitreous luster, no cleavage, and no crystal structure; exact identification is uncertain from the image alone because black agate, onyx, or tourmaline are possible.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, usually during Quaternary volcanic eruptions. Common sources include Mexico, the western United States, Iceland, Armenia, and Japan.
Uses & applications
Used for ornamental carvings, cabochons, beads, knife blades, and decorative objects; high-quality polished pieces are inexpensive but attractive.
Geological facts
Obsidian can produce exceptionally sharp edges when fractured and was historically used for tools and arrowheads. Snowflake, rainbow, and mahogany varieties contain crystalline or colored inclusions.
Field identification & locations
Check for curved conchoidal fracture, glass-like luster, and a sharp edge; a steel file may scratch it slightly. Black agate is usually more translucent and banded, while onyx commonly shows parallel color bands.
