igneous
Dark basalt or vesicular volcanic rock
Basalt; predominantly plagioclase, pyroxene, and commonly olivine, with possible iron-oxide alteration
AI-generated identification · may be incorrectFine-grained, dark gray to black, dull to weakly submetallic surface; appears dense with scattered pale mineral grains and weathered patches. Basalt typically has Mohs hardness 5–6, no visible cleavage, and specific gravity about 2.8–3.0.
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Physical properties
Fine-grained, dark gray to black, dull to weakly submetallic surface; appears dense with scattered pale mineral grains and weathered patches. Basalt typically has Mohs hardness 5–6, no visible cleavage, and specific gravity about 2.8–3.0.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic to Recent volcanic activity; its exact age cannot be determined from the photograph.
Uses & applications
Used extensively as crushed aggregate, road base, riprap, railway ballast, building stone, and occasionally in landscaping or polished specimens; ordinary pieces have little jewelry value.
Geological facts
Basalt is the most widespread volcanic rock on Earth and forms much of the oceanic crust. The elongated shape may reflect human shaping or a weathered fragment, so it should not be assumed to be a natural boulder form.
Field identification & locations
Identify with a gray-black fine-grained texture, high density, and lack of obvious quartz crystals; a streak test, fresh broken surface, magnet response, and hand lens would help distinguish basalt from slag or dark metamorphic rock. Common worldwide, especially volcanic regions and oceanic crust exposures.
