igneous
Porphyritic basalt (likely olivine basalt)
Basalt; fine-grained plagioclase–pyroxene matrix with possible olivine phenocrysts
AI-generated identification · may be incorrectDark gray-green, rounded and weathered, with numerous pale mineral grains; typically dull to faintly vitreous, hardness about 6, no visible cleavage, fine-grained crystalline texture, and specific gravity around 2.8–3.1.
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Physical properties
Dark gray-green, rounded and weathered, with numerous pale mineral grains; typically dull to faintly vitreous, hardness about 6, no visible cleavage, fine-grained crystalline texture, and specific gravity around 2.8–3.1.
Formation & geological history
An extrusive igneous rock formed when mafic lava cooled rapidly at or near Earth’s surface. Basalt commonly ranges from Precambrian to modern age and forms volcanic flows, oceanic crust, and many shield volcanoes.
Uses & applications
Used as crushed aggregate, road metal, railroad ballast, building stone, and occasionally polished as a decorative specimen. Common pieces have little commercial value unless unusually attractive or historically significant.
Geological facts
The pale specks are likely plagioclase or altered olivine; exact identification requires a close examination or testing. Its rounded shape indicates transport and abrasion, rather than its original lava-flow form.
Field identification & locations
Common in volcanic regions, oceanic islands, and areas with ancient lava flows, including Hawaii, Iceland, India, and the Pacific Northwest. A fresh broken surface, magnetic response, and lack of fizz with dilute acid support basalt identification.
