igneous
Likely weathered basalt or andesite fragment
Fine-grained mafic to intermediate volcanic rock; commonly plagioclase ± pyroxene ± olivine
AI-generated identification · may be incorrectDark brown-black, dull to faintly earthy luster, massive and fine-grained with irregular fracture; estimated Mohs hardness 5–6 and specific gravity about 2.7–3.1. The image is low-resolution, so vesicles, crystals, and exact mineralogy are not discernible.
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Physical properties
Dark brown-black, dull to faintly earthy luster, massive and fine-grained with irregular fracture; estimated Mohs hardness 5–6 and specific gravity about 2.7–3.1. The image is low-resolution, so vesicles, crystals, and exact mineralogy are not discernible.
Formation & geological history
Formed when lava cooled relatively quickly at or near Earth’s surface, producing microscopic crystals; its precise age and volcanic source cannot be determined from the photograph.
Uses & applications
Typically used as crushed aggregate, roadstone, riprap, and occasionally dimension stone; an ordinary fragment generally has negligible jewelry or collector value.
Geological facts
Basalt is the most abundant volcanic rock in oceanic crust, while andesite commonly forms at volcanic arcs above subduction zones. Brown coloration likely reflects surface weathering and iron oxidation.
Field identification & locations
Look for a dense, dark, fine-grained texture, hardness sufficient to scratch glass, and possible tiny plagioclase crystals or vesicles; magnetism can support a basalt identification. Common worldwide in volcanic terrains.
