igneous
Weathered basalt (possibly iron-stained volcanic rock)
Basalt; predominantly plagioclase, pyroxene, and possibly olivine, with iron-oxide staining
AI-generated identification · may be incorrectDark gray to black, fine-grained and generally dull, with yellow-brown surface staining from limonite/goethite. Basalt hardness is about Mohs 6, with massive texture, no obvious cleavage, and specific gravity around 2.8–3.0.
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Physical properties
Dark gray to black, fine-grained and generally dull, with yellow-brown surface staining from limonite/goethite. Basalt hardness is about Mohs 6, with massive texture, no obvious cleavage, and specific gravity around 2.8–3.0.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, producing tiny crystals; the specimen’s rounded shape reflects weathering and abrasion. Its exact geological age and location cannot be determined from the image.
Uses & applications
Commonly used as crushed aggregate, road material, railway ballast, riprap, and occasionally dimension stone; attractive pieces may be collected but have little gem value.
Geological facts
The yellow patches are likely weathering products rather than native yellow minerals. Similar dark rocks can include andesite, scoria, or altered basalt, which require a fresh surface or testing for confident identification.
Field identification & locations
Look for a dense, dark, fine-grained rock that does not fizz with dilute acid and commonly attracts weakly to a magnet if magnetite is present. Common worldwide, especially in volcanic regions; approximate value is $1–$10 for a hand specimen, usually under $20 unless unusually attractive or locality-specific.
