igneous
Dark basalt pebble
Basalt; primarily plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectFine-grained, dark gray, aphanitic, and generally dull to slightly glassy; hardness about 5–6 Mohs, with little or no visible cleavage. The pale markings may be weathering, mineral veins, or surface residue; exact identification is limited by the image.
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Physical properties
Fine-grained, dark gray, aphanitic, and generally dull to slightly glassy; hardness about 5–6 Mohs, with little or no visible cleavage. The pale markings may be weathering, mineral veins, or surface residue; exact identification is limited by the image.
Formation & geological history
Extrusive volcanic rock formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic activity. Rounded shape indicates transport and abrasion in a stream, beach, or gravel deposit.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and occasionally decorative stone. Common specimens have little jewelry or collector value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify by its dense, fine-grained dark appearance, lack of large crystals, and typically high density; a streak, fresh broken surface, and magnet test can help. Common worldwide, especially volcanic regions and river or beach gravels.
