igneous
Rounded volcanic rock, likely basalt
Basalt (fine-grained mafic igneous rock; commonly plagioclase, pyroxene, and olivine)
AI-generated identification · may be incorrectDark brown-black, weathered surface with dull to slight vitreous luster; likely fine-grained and massive, with Mohs hardness about 5–6 and specific gravity roughly 2.8–3.1. The rounded shape indicates water abrasion; pale specks may be feldspar or weathering products.
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Physical properties
Dark brown-black, weathered surface with dull to slight vitreous luster; likely fine-grained and massive, with Mohs hardness about 5–6 and specific gravity roughly 2.8–3.1. The rounded shape indicates water abrasion; pale specks may be feldspar or weathering products.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes; the exact age cannot be determined from the photograph. Its smooth form likely developed in a river, beach, or glacial setting.
Uses & applications
Used as aggregate, roadstone, riprap, paving stone, and occasionally decorative stone; this small weathered specimen has little commercial value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. Brown coloration commonly reflects oxidation of iron-bearing minerals during weathering.
Field identification & locations
Identify by its dense, dark, fine-grained appearance, lack of obvious layering, and inability to scratch it with a fingernail; a steel nail may scratch weathered areas. Common worldwide in volcanic terrains and stream gravels.
