igneous
Rounded basalt pebble
Basalt, a fine-grained mafic volcanic rock (mainly plagioclase and pyroxene)
AI-generated identification · may be incorrectDark gray-green, smoothly water-worn, and very fine-grained with a dull to slightly satin luster; typical hardness is about 5–6 Mohs, with no visible cleavage or large crystals. The circular pale spot may be a weathered vesicle, mineral inclusion, or impact mark.
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Physical properties
Dark gray-green, smoothly water-worn, and very fine-grained with a dull to slightly satin luster; typical hardness is about 5–6 Mohs, with no visible cleavage or large crystals. The circular pale spot may be a weathered vesicle, mineral inclusion, or impact mark.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. The pebble’s precise age and source cannot be determined from the photograph.
Uses & applications
Common basalt is used as crushed aggregate, road stone, concrete material, railroad ballast, and building stone; rounded pebbles are mainly decorative or collectible and usually have little commercial value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its greenish color can reflect weathering or alteration of iron-bearing minerals.
Field identification & locations
Identify it by its heavy, fine-grained dark appearance, lack of obvious layering, and inability to scratch easily with a steel knife; a streak, magnetism, and acid test can help distinguish it from other pebbles. Common worldwide, especially on volcanic coasts and riverbeds.
