igneous
Likely weathered basalt or mafic volcanic rock
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine or iron oxides
AI-generated identification · may be incorrectDark reddish-brown, fine-grained, dull to weakly earthy luster, with no visible cleavage; fresh basalt has Mohs hardness about 6, though weathering softens the surface. The elongated shape may be natural erosion or human shaping, but the image alone cannot confirm a specific specimen.
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Physical properties
Dark reddish-brown, fine-grained, dull to weakly earthy luster, with no visible cleavage; fresh basalt has Mohs hardness about 6, though weathering softens the surface. The elongated shape may be natural erosion or human shaping, but the image alone cannot confirm a specific specimen.
Formation & geological history
Basalt forms when low-silica lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. It may range from ancient Precambrian rocks to geologically recent lava flows.
Uses & applications
Used widely as crushed aggregate, road base, railroad ballast, riprap, and building stone; attractive polished pieces may be sold as decorative specimens. Typical loose collector value is about $5–$25, rising to roughly $30–$100 if unusually shaped, polished, or locality-labeled.
Geological facts
Its reddish coloration likely reflects oxidation of iron-bearing minerals during weathering. Basalt is among Earth’s most abundant rocks and forms much of the ocean floor.
Field identification & locations
Look for a dense, fine-grained texture, dark color, high specific gravity, and lack of quartz; a streak, magnetic response, and acid test can help distinguish it from altered volcanic rocks. Common worldwide, especially Hawaii, Iceland, India, the Columbia River region, and oceanic islands.
