igneous
Dark river-worn stone, likely basalt
Basalt; primarily plagioclase feldspar and pyroxene, with possible olivine and magnetite
AI-generated identification · may be incorrectVery fine-grained, dark gray to black, dull to subtly waxy luster, with no visible crystals or cleavage; Mohs hardness about 5–6 and specific gravity roughly 2.8–3.0. The smooth, rounded surface indicates prolonged abrasion in water or sediment.
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Physical properties
Very fine-grained, dark gray to black, dull to subtly waxy luster, with no visible crystals or cleavage; Mohs hardness about 5–6 and specific gravity roughly 2.8–3.0. The smooth, rounded surface indicates prolonged abrasion in water or sediment.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and ancient lava flows. The specimen’s exact age cannot be determined from the image.
Uses & applications
Used as aggregate, road stone, building stone, riprap, and occasionally as polished decorative material or a massage stone. Ordinary pieces have little jewelry value.
Geological facts
Basalt is the most widespread volcanic rock on Earth and commonly contains microscopic crystals even when it appears glassy. This specimen may also be another dark fine-grained rock, such as andesite, because mineral details are not visible.
Field identification & locations
Look for uniformly dark color, fine grain, high density, and lack of fizz with dilute acid; a streak test is usually gray. Common locations include volcanic terrains and riverbeds worldwide.
