igneous
Likely basalt (dark volcanic rock)
Basalt; fine-grained mafic rock composed chiefly of plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray-green, dull to weakly submetallic, massive and fine-grained; Mohs hardness about 5–6, with poorly visible crystals and no obvious cleavage. The rounded shape appears naturally water-worn rather than crystalline.
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Physical properties
Dark gray-green, dull to weakly submetallic, massive and fine-grained; Mohs hardness about 5–6, with poorly visible crystals and no obvious cleavage. The rounded shape appears naturally water-worn rather than crystalline.
Formation & geological history
Forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic flows, and flood-basalt provinces. Individual specimens may be geologically young or millions to billions of years old; age cannot be determined visually.
Uses & applications
Used as crushed aggregate, road stone, railway ballast, riprap, and occasionally building or landscaping stone; generally not used as a gemstone.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Green coloration may reflect olivine, chlorite, epidote, or weathering, but confirmation requires testing or microscopy.
Field identification & locations
Look for a dense, dark, fine-grained rock with tiny pale feldspar specks, high specific gravity, and no acid fizz; a streak, magnetism, thin section, or chemical test can distinguish it from greenstone, gabbro, or slag. Common worldwide, especially volcanic regions and streambeds.
