igneous
Rounded dark river pebble, likely basalt
Basalt; fine-grained mafic volcanic rock, typically plagioclase + pyroxene ± olivine
AI-generated identification · may be incorrectUsually black, gray, brown, or greenish with a dull to subtly waxy luster; hardness about 6–6.5 Mohs, dense, tough, and non-cleaving. The specimen appears water-rounded and weathered, with no visible crystals at this scale.
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Physical properties
Usually black, gray, brown, or greenish with a dull to subtly waxy luster; hardness about 6–6.5 Mohs, dense, tough, and non-cleaving. The specimen appears water-rounded and weathered, with no visible crystals at this scale.
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 precise age and origin cannot be determined from the photograph.
Uses & applications
Used as aggregate, roadstone, riprap, railroad ballast, and building stone; attractive polished pieces are sold as inexpensive decorative specimens. This small pebble has minimal commercial value.
Geological facts
Basalt is the most common volcanic rock and makes up much of the ocean floor. Weathering can produce brown iron-oxide coatings and obscure its original black color.
Field identification & locations
Look for a fine-grained, heavy, dark rock that may attract a weak magnet if iron-rich; a fresh broken surface is typically dark gray and uniform. Common in stream beds and beaches downstream from volcanic or mafic terrains.
