igneous
Dark volcanic rock, likely basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray to black, massive, fine-grained, and dull to subtly vitreous; typically Mohs 5–6, with no visible cleavage and specific gravity around 2.8–3.1. The smooth surface may reflect weathering or natural rounding, though the image does not show diagnostic crystals or vesicles.
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Physical properties
Dark gray to black, massive, fine-grained, and dull to subtly vitreous; typically Mohs 5–6, with no visible cleavage and specific gravity around 2.8–3.1. The smooth surface may reflect weathering or natural rounding, though the image does not show diagnostic crystals or vesicles.
Formation & geological history
Basalt forms when low-silica magma rapidly cools at or near Earth’s surface, commonly in lava flows, flood-basalt provinces, oceanic crust, and volcanic islands. Its age cannot be determined visually and may range from modern to billions of years.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, paving stone, and building stone; attractive polished pieces are occasionally sold as decorative specimens.
Geological facts
Its dark color and fine texture are consistent with basalt, but dense andesite or another dark volcanic rock is also possible from the photograph alone.
Field identification & locations
Look for a fine crystalline texture, high density, hardness sufficient to scratch glass, and occasional tiny feldspar crystals or gas bubbles. Common worldwide, especially in volcanic regions; value is usually about $5–$30 for an ordinary specimen, or $30–$100+ if unusually fresh, vesicular, or display-quality.
