igneous
Tumbled black stone, likely black obsidian or slag glass
Obsidian (volcanic glass), tentatively; composition commonly rhyolitic, rich in silica
AI-generated identification · may be incorrectBlack, glossy to vitreous surface with rounded tumbled form and possible conchoidal texture; Mohs hardness about 5–5.5, no cleavage, amorphous rather than crystalline, specific gravity about 2.35–2.65. The visible pits and highly rounded shape could also indicate industrial slag or coated stone, which cannot be confirm…
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Physical properties
Black, glossy to vitreous surface with rounded tumbled form and possible conchoidal texture; Mohs hardness about 5–5.5, no cleavage, amorphous rather than crystalline, specific gravity about 2.35–2.65. The visible pits and highly rounded shape could also indicate industrial slag or coated stone, which cannot be confirmed from the image.
Formation & geological history
Obsidian forms when silica-rich lava cools rapidly, preventing crystal growth, generally during relatively young volcanic eruptions from the Cenozoic to recent era. Common sources include volcanic regions of the western United States, Mexico, Japan, Iceland, and the Mediterranean.
Uses & applications
Used for ornamental objects, beads, cabochons, carvings, and historically extremely sharp tools and weapons. Common tumbled pieces are inexpensive.
Geological facts
A fresh fracture of obsidian is sharply conchoidal and can produce edges sharper than surgical steel; its glassy appearance distinguishes it from most crystalline black rocks.
Field identification & locations
Check for uniformly glassy texture, conchoidal fracture, and lack of visible grains; a streak, density, and magnet test can help distinguish slag. Common tumbled specimens typically sell for $2–$15; unusual varieties or polished larger pieces may reach $20–$75.
