igneous
Tumbled black stone, likely obsidian
Volcanic glass; chiefly amorphous silica (SiO₂)
AI-generated identification · may be incorrectBlack, smoothly polished, vitreous luster, no visible crystals or cleavage; Mohs hardness about 5–5.5 and specific gravity about 2.3–2.6. The rounded shape indicates tumbling; the image alone cannot exclude black jasper, basalt, or dyed material.
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Physical properties
Black, smoothly polished, vitreous luster, no visible crystals or cleavage; Mohs hardness about 5–5.5 and specific gravity about 2.3–2.6. The rounded shape indicates tumbling; the image alone cannot exclude black jasper, basalt, or dyed material.
Formation & geological history
Obsidian forms when silica-rich lava cools so rapidly that crystals cannot grow, commonly in volcanic settings from the Cenozoic to present. It is widespread near rhyolitic volcanoes, especially in western North America, Mexico, Iceland, and the Mediterranean.
Uses & applications
Used for ornamental objects, cabochons, beads, carvings, and historically exceptionally sharp cutting tools. Tumbled pieces have modest decorative and collecting value.
Geological facts
Fresh obsidian can fracture conchoidally into edges sharper than surgical steel, though it is brittle. Snowflake obsidian contains white cristobalite inclusions; none are clearly visible here.
Field identification & locations
Check for uniform glassy black color, conchoidal fracture, and lack of granular texture; a streak test is usually white or colorless. Confirmatory identification may require microscopy or density testing because polished black stones are easily confused.
