igneous
Rounded basalt pebble
Basalt, aphanitic mafic volcanic rock; primarily plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectDark gray-green, fine-grained, dull to weakly granular, and strongly rounded by abrasion; Mohs hardness about 5–6, no visible cleavage, typically dense (specific gravity ~2.8–3.0). The image does not show enough detail for confident mineral-level identification.
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Physical properties
Dark gray-green, fine-grained, dull to weakly granular, and strongly rounded by abrasion; Mohs hardness about 5–6, no visible cleavage, typically dense (specific gravity ~2.8–3.0). The image does not show enough detail for confident mineral-level identification.
Formation & geological history
Basalt forms when low-silica magma rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces; its age may range from modern to billions of years. The rounded shape indicates transport in a stream, beach, or glacial sediment.
Uses & applications
Used as crushed aggregate, road base, riprap, railway ballast, concrete aggregate, and occasionally as landscaping or polishing material; this small pebble has negligible commercial value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify it by its fine texture, dark gray color, high density, and lack of obvious layers or large crystals; a streak, magnetism test, and fresh fracture would improve confidence. Common worldwide, especially volcanic and gravel deposits.
