igneous
Likely basalt (possibly weathered volcanic rock)
Basalt; predominantly plagioclase feldspar and pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectDark gray to brown, fine-grained and granular, with scattered pale mineral crystals and weathered surfaces; typically Mohs 6, dull to subvitreous luster, no obvious cleavage, and specific gravity about 2.8–3.0. The visible pale patches could also indicate alteration or a basaltic breccia.
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Physical properties
Dark gray to brown, fine-grained and granular, with scattered pale mineral crystals and weathered surfaces; typically Mohs 6, dull to subvitreous luster, no obvious cleavage, and specific gravity about 2.8–3.0. The visible pale patches could also indicate alteration or a basaltic breccia.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during volcanic eruptions; individual specimens may range from ancient Precambrian flows to recent lava. The exact age and locality cannot be determined from the image.
Uses & applications
Used extensively as crushed aggregate, road stone, railway ballast, building stone, and landscaping material; attractive polished pieces may be sold as decorative stone. This irregular specimen has little commercial value without provenance.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows, dikes, and pillow lavas. Its dark color reflects abundant ferromagnesian minerals and iron-rich glass or oxides.
Field identification & locations
Identify it by its dark, heavy, fine-grained texture and lack of layering; a streak test, hardness test, and hand lens would help distinguish it from andesite or altered metamorphic rock. Common worldwide, especially in volcanic provinces and oceanic crust.
