igneous
Tumbled basalt (vesicular volcanic rock)
Basalt, predominantly plagioclase and pyroxene with possible olivine and iron oxides
AI-generated identification · may be incorrectHardness about 5.5–6.5 Mohs; dark brown to black, weathered bronze-brown surface, dull to weakly vitreous luster, fine-grained interlocking crystals, and irregular vesicles or cavities. Cleavage is generally absent as a whole rock; specific gravity is about 2.8–3.0.
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Physical properties
Hardness about 5.5–6.5 Mohs; dark brown to black, weathered bronze-brown surface, dull to weakly vitreous luster, fine-grained interlocking crystals, and irregular vesicles or cavities. Cleavage is generally absent as a whole rock; specific gravity is about 2.8–3.0.
Formation & geological history
Extrusive mafic magma cooled rapidly at or near Earth’s surface, likely forming in a lava flow; the exact age cannot be determined visually and may range from recent to billions of years. The rounded shape appears produced by stream, beach, or human tumbling.
Uses & applications
Used as aggregate, roadstone, riprap, railway ballast, construction stone, and landscaping material; attractive pieces are sold as curios or polishing/tumbling rough.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms from decompression melting of the mantle. Vesicles formed when dissolved volcanic gases escaped from the cooling lava.
Field identification & locations
Identify by its heavy, dark, fine-grained texture and lack of visible large crystals; a streak test is usually gray, and it should not fizz with dilute hydrochloric acid. Common worldwide, especially oceanic islands, volcanic plateaus, and ancient lava fields.
