igneous
Tumbled black stone, likely black obsidian
Obsidian (volcanic glass; composition typically rhyolitic)
AI-generated identification · may be incorrectAppears glossy, opaque, and black with a smooth tumbled surface. Obsidian has no crystal structure or cleavage and is about 5–5.5 on the Mohs scale; the photo alone cannot confirm the identification.
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Physical properties
Appears glossy, opaque, and black with a smooth tumbled surface. Obsidian has no crystal structure or cleavage and is about 5–5.5 on the Mohs scale; the photo alone cannot confirm the identification.
Formation & geological history
Forms when silica-rich lava cools rapidly, preventing crystals from growing; it is generally geologically young compared with many crystalline rocks.
Uses & applications
Used for decorative objects, beads, jewelry, and historically for exceptionally sharp cutting tools.
Geological facts
Obsidian breaks with a distinctive curved, shell-like fracture and can produce edges sharper than surgical steel, though those edges are brittle.
Field identification & locations
Common sources include volcanic regions such as Mexico, the western United States, Iceland, and Japan. A glassy sheen and conchoidal fractures on a broken surface support identification; the pictured piece is too polished for a definitive ID.
