igneous
Tumbled basalt (likely vesicular basalt)
Basalt, a mafic volcanic rock composed mainly of plagioclase feldspar and pyroxene, commonly with olivine and iron oxides
AI-generated identification · may be incorrectDark gray-black, fine-grained, opaque, and typically dull to submetallic; Mohs hardness about 5–6, massive with no visible cleavage. The rounded, glossy surface indicates human tumbling/polishing, while small pits may be vesicles or surface inclusions.
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Physical properties
Dark gray-black, fine-grained, opaque, and typically dull to submetallic; Mohs hardness about 5–6, massive with no visible cleavage. The rounded, glossy surface indicates human tumbling/polishing, while small pits may be vesicles or surface inclusions.
Formation & geological history
Basalt forms when low-silica, iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in volcanic islands, flood-basalt provinces, and oceanic crust. Most specimens are geologically young to ancient; the image alone cannot establish age.
Uses & applications
Used as aggregate, road stone, building stone, ballast, and decorative landscaping material; tumbled pieces are sold for crafts and collections. Typical value is modest.
Geological facts
Its high iron content produces the dark color and may cause weak magnetism, especially if magnetite-rich. A streak test is usually gray to black, but polished surfaces can appear unusually reflective.
Field identification & locations
Common worldwide, especially near volcanic regions; likely sources include Hawaii, Iceland, India, Mexico, and the western United States. Check for fine texture, heft, lack of glassy conchoidal fracture, and weak magnetic response; exact identification would require testing.
