igneous
Dark volcanic rock, likely basalt
Basalt; fine-grained mafic silicate rock dominated by plagioclase and pyroxene, possibly with olivine
AI-generated identification · may be incorrectTypically Mohs 5–6, dark gray to black, dull to subtly glassy, and fine-grained with crystals too small to see; the specimen appears weathered and may contain small lighter mineral grains. Cleavage is not apparent; specific gravity is commonly about 2.8–3.0.
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Physical properties
Typically Mohs 5–6, dark gray to black, dull to subtly glassy, and fine-grained with crystals too small to see; the specimen appears weathered and may contain small lighter mineral grains. Cleavage is not apparent; specific gravity is commonly about 2.8–3.0.
Formation & geological history
Forms when basaltic lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and lava flows; most such beach stones are geologically young relative to Earth’s 4.54-billion-year history.
Uses & applications
Used widely as crushed aggregate, road base, riprap, concrete material, and building stone; ordinary beach fragments have negligible jewelry or collector value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify it by its dark color, heavy feel, fine texture, and lack of visible layering; a streak, magnetism test, and fresh broken surface help distinguish it from slag or other dark rocks. Common on volcanic coastlines and glacial beaches.
