igneous
Black basalt pebble
Basalt; fine-grained mafic volcanic rock, mainly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectTypically Mohs 5–6, black to dark gray, dull to slightly vitreous, and very fine-grained; individual crystals are usually invisible. The specimen appears water-worn and may show a subtle fracture or weathered surface.
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Physical properties
Typically Mohs 5–6, black to dark gray, dull to slightly vitreous, and very fine-grained; individual crystals are usually invisible. The specimen appears water-worn and may show a subtle fracture or weathered surface.
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. It may range from recent volcanic material to rocks hundreds of millions of years old; age cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and occasionally dimension stone or landscaping material. Smooth pebbles are common inexpensive décor and teaching specimens.
Geological facts
Its dark color reflects abundant ferromagnesian minerals and low silica relative to granite. Some basalt is magnetic because it contains magnetite.
Field identification & locations
Identify it by its dark color, heavy feel, fine texture, and lack of visible quartz or large crystals; a streak test is usually gray, and a magnet may show weak attraction. Common worldwide, especially volcanic islands and river or beach gravels.
