igneous
Rounded gray pebble, likely basalt
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectTypically dark gray to bluish gray, dull to subtly glassy, dense, and fine-grained; hardness about 6 on the Mohs scale, with no visible cleavage and generally high specific gravity (~2.8–3.0). The specimen appears water-worn or tumbled.
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Physical properties
Typically dark gray to bluish gray, dull to subtly glassy, dense, and fine-grained; hardness about 6 on the Mohs scale, with no visible cleavage and generally high specific gravity (~2.8–3.0). The specimen appears water-worn or tumbled.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and flood-basalt provinces. Its exact age cannot be determined from the image; basalt ranges from Archean to modern.
Uses & applications
Used as crushed aggregate, road base, railway ballast, building stone, and occasionally decorative pavers or lapidary material. This small pebble has negligible commercial value.
Geological facts
Basalt is the most common volcanic rock and makes up much of the ocean floor. Some basalt contains vesicles, olivine crystals, or magnetic iron minerals.
Field identification & locations
Identify it by its uniform fine texture, dark gray color, heft, and lack of visible quartz or large crystals; a streak, magnetism test, and fresh fracture would help confirm it. Common sources include beaches, river gravels, volcanic terrains, and landscaping stone.
