igneous
Dark fine-grained volcanic rock, probably basalt
Basalt; primarily plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, with a dull to weakly glassy luster and no visible large crystals; Mohs hardness about 6, with poor or absent cleavage and specific gravity around 2.8–3.0. The specimen appears weathered and may contain a lighter mineral vein or alteration zone.
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Physical properties
Typically dark gray to black, fine-grained, with a dull to weakly glassy luster and no visible large crystals; Mohs hardness about 6, with poor or absent cleavage and specific gravity around 2.8–3.0. The specimen appears weathered and may contain a lighter mineral vein or alteration zone.
Formation & geological history
Basalt forms when low-silica magma rapidly cools at or near Earth’s surface, commonly in lava flows, oceanic crust, and volcanic islands. Most examples range from recent volcanic ages to billions of years old; this hand specimen’s age cannot be determined visually.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, riprap, and dimension stone; attractive polished pieces may be sold as decorative stone, though this specimen has little gem value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its exact identification should be confirmed with a streak, magnetism, acid, and thin-section or chemical testing.
Field identification & locations
Look for a heavy, dark, compact rock with tiny interlocking crystals; vesicles, magnetic response, or olivine spots support basalt identification. Common locations include volcanic regions and ancient lava terrains worldwide.
