igneous
Dark gray river pebble, likely basalt
Basalt; chiefly plagioclase feldspar, pyroxene, and minor olivine or magnetite
AI-generated identification · may be incorrectFine-grained, dark gray to black, dull to faintly waxy luster, with no visible crystals or cleavage; Mohs hardness about 5–6 and specific gravity about 2.8–3.0. The smooth, rounded shape indicates water abrasion, while the blue-gray patch may reflect weathering or surface coating.
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Physical properties
Fine-grained, dark gray to black, dull to faintly waxy luster, with no visible crystals or cleavage; Mohs hardness about 5–6 and specific gravity about 2.8–3.0. The smooth, rounded shape indicates water abrasion, while the blue-gray patch may reflect weathering or surface coating.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its exact age cannot be determined from the image; it may range from recent to billions of years old.
Uses & applications
Used as crushed aggregate, road stone, riprap, ballast, and occasionally as a decorative or polished stone. This small specimen has minimal commercial value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and makes up much of the ocean floor. Its fine texture results from rapid cooling that prevents large crystals from forming.
Field identification & locations
Identify it by its dense, dark, fine-grained texture and lack of obvious layering or large crystals; a streak test is typically gray, and it commonly does not fizz with dilute hydrochloric acid. Common sources include streambeds and volcanic regions worldwide.
