igneous
Tumbled basalt or dark volcanic rock
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine or iron oxides
AI-generated identification · may be incorrectFine-grained, dark gray to brown-black, locally rusty from oxidation, with a dull to subtly vitreous luster; Mohs hardness about 5–6, massive texture, no visible cleavage, and specific gravity roughly 2.8–3.0.
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Physical properties
Fine-grained, dark gray to brown-black, locally rusty from oxidation, with a dull to subtly vitreous luster; Mohs hardness about 5–6, massive texture, no visible cleavage, and specific gravity roughly 2.8–3.0.
Formation & geological history
Basalt forms when low-silica lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. The rounded shape and smooth surface indicate natural stream or beach abrasion, possibly with light polishing.
Uses & applications
Used as aggregate, road stone, riprap, paving, and basalt fiber; attractive pieces may be sold as tumbled stones or paperweights. This specimen has modest decorative value.
Geological facts
Rusty brown areas likely reflect weathered iron-bearing minerals, while tiny pale flecks may be feldspar or alteration products. The image does not show diagnostic crystals clearly enough for a precise variety identification.
Field identification & locations
Identify by its heavy feel, dark fine-grained appearance, lack of obvious layering, and resistance to scratching with a steel knife; it should not fizz with dilute acid. Common worldwide, especially near volcanic regions and basaltic rivers or beaches.
