igneous
Porphyritic basalt (likely amygdaloidal basalt)
Basalt, a mafic volcanic rock; predominantly plagioclase and pyroxene, commonly with olivine and secondary zeolite/copper minerals
AI-generated identification · may be incorrectDark gray-black, fine-grained groundmass with pale pink, green, and blue-green inclusions or weathering patches; typically Mohs 5–6, dull to subvitreous luster, and no obvious cleavage. The rounded shape indicates water-worn transport, while some light patches may be vesicles filled by secondary minerals.
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Physical properties
Dark gray-black, fine-grained groundmass with pale pink, green, and blue-green inclusions or weathering patches; typically Mohs 5–6, dull to subvitreous luster, and no obvious cleavage. The rounded shape indicates water-worn transport, while some light patches may be vesicles filled by secondary minerals.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, generally in volcanic flows from the Cenozoic or older eras; cavities can later fill with zeolites, calcite, chlorite, or copper minerals. Exact age and locality cannot be determined from the photograph.
Uses & applications
Used as aggregate, roadstone, riprap, landscaping stone, and occasionally as a polished decorative material; mineral-filled examples are collected. Typical value is about $5–$30, or $30–$100 if the inclusions are unusually attractive or demonstrably copper-bearing.
Geological facts
The green-blue patches could represent chlorite, chrysocolla, malachite, or weathered copper sulfides, but visual identification alone is insufficient. Basalt is the most abundant volcanic rock on oceanic crust and commonly forms columnar jointing.
Field identification & locations
Look for a dense dark matrix, fine grain, magnetic response, and pale mineral-filled cavities; a streak, acid test, hardness test, and locality information would help confirm it. Common in volcanic terrains and stream or beach gravels, including the Pacific Northwest, Hawaii, Iceland, and many continental basalt provinces.
