igneous
Rounded basaltic river cobble
Basalt, a fine-grained mafic volcanic rock; chiefly plagioclase feldspar, pyroxene, and often olivine
AI-generated identification · may be incorrectDark gray-brown, fine-grained, and typically dull to slightly satin in luster; hardness about 5–6 Mohs, massive texture, no visible cleavage, and specific gravity about 2.8–3.1. Its rounded, smooth shape indicates prolonged water or sediment abrasion.
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Physical properties
Dark gray-brown, fine-grained, and typically dull to slightly satin in luster; hardness about 5–6 Mohs, massive texture, no visible cleavage, and specific gravity about 2.8–3.1. Its rounded, smooth shape indicates prolonged water or sediment abrasion.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic activity; the exact age cannot be determined visually. It was later transported and rounded in a stream, beach, or glacial environment.
Uses & applications
Used extensively as crushed aggregate, road stone, riprap, concrete aggregate, and landscaping material; attractive polished pieces may be sold as decorative or jewelry stones.
Geological facts
Basalt is the most common volcanic rock on Earth and forms much of the oceanic crust. The specimen may alternatively be a closely related fine-grained diabase or andesite, but its dark, uniform appearance is most consistent with basalt.
Field identification & locations
Look for a dense, fine-grained, dark rock that is heavier than similarly sized felsic stones and lacks obvious quartz crystals; a streak, magnetism test, and thin-section examination can improve identification. Common in volcanic regions and gravel deposits worldwide.
