igneous
Polished black pebble, possibly obsidian
Volcanic glass (obsidian), likely; identification is tentative from the image
AI-generated identification · may be incorrectBlack, very glossy vitreous luster with a smoothly tumbled surface and conchoidal-looking reflections. Obsidian hardness is about 5–5.5 Mohs, lacks a crystal structure and cleavage, and has specific gravity around 2.3–2.6; the bright streaks may also indicate polished common stone or glass.
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Physical properties
Black, very glossy vitreous luster with a smoothly tumbled surface and conchoidal-looking reflections. Obsidian hardness is about 5–5.5 Mohs, lacks a crystal structure and cleavage, and has specific gravity around 2.3–2.6; the bright streaks may also indicate polished common stone or glass.
Formation & geological history
Obsidian forms when silica-rich lava cools rapidly, preventing crystallization, usually in volcanic regions younger than roughly 20 million years. The specimen appears human-polished or tumbled.
Uses & applications
Used for ornamental objects, jewelry, cabochons, cutting tools, and collectibles; quality black obsidian pieces are inexpensive.
Geological facts
Obsidian is technically natural volcanic glass rather than a crystalline mineral. Snowflake, rainbow, sheen, and mahogany varieties arise from inclusions or optical effects.
Field identification & locations
Common sources include Mexico, the western United States, Armenia, Iceland, and Japan. Check for natural conchoidal fracture, a glassy interior, and absence of bubbles or mold marks; a streak test is usually inconclusive. It could be man-made glass, which would not qualify as geological material.