igneous
Tumbled basalt (likely)
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase, pyroxene, and olivine
AI-generated identification · may be incorrectDark gray to black, smoothly polished, with a dull to subtly silky luster; typically massive and nonfoliated, Mohs hardness about 6. Specific gravity is approximately 2.8–3.0; individual crystals are too fine to see clearly.
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Physical properties
Dark gray to black, smoothly polished, with a dull to subtly silky luster; typically massive and nonfoliated, Mohs hardness about 6. Specific gravity is approximately 2.8–3.0; individual crystals are too fine to see clearly.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. Its age may range from recent eruptions to billions of years, but this tumbled piece cannot be dated visually.
Uses & applications
Used as aggregate, road stone, riprap, railroad ballast, and occasionally dimension stone; polished pieces are inexpensive decorative or grounding stones.
Geological facts
Basalt is the most abundant volcanic rock on Earth and commonly forms oceanic crust. The rounded shape and glossy surface indicate mechanical tumbling and polishing rather than natural weathering.
Field identification & locations
Identify it by its uniformly dark, dense appearance, fine grain, lack of obvious layering, and inability to scratch it easily with a steel knife; a streak test is usually gray to black. Common sources include Hawaii, Iceland, India, Oregon, and many volcanic regions.
