igneous
Dark rounded pebble, likely basalt
Basalt — fine-grained mafic volcanic rock, mainly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectDark gray-brown, dull to weakly glassy, fine-grained and rounded from abrasion; hardness about 6 Mohs, no visible cleavage, typically dense (specific gravity ~2.8–3.0). Identification is tentative because the image lacks scale and close detail.
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Physical properties
Dark gray-brown, dull to weakly glassy, fine-grained and rounded from abrasion; hardness about 6 Mohs, no visible cleavage, typically dense (specific gravity ~2.8–3.0). Identification is tentative because the image lacks scale and close detail.
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. Individual pieces may be geologically ancient or relatively recent.
Uses & applications
Used widely as crushed aggregate, roadstone, riprap, railway ballast, and building stone; attractive polished pieces are used decoratively but usually have low gem value.
Geological facts
Basalt is the most common volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant ferromagnesian minerals and relatively low silica content.
Field identification & locations
Look for a uniformly fine texture, high density, dark color, and lack of obvious layers; a magnet may show weak attraction if magnetite is present. Common worldwide, especially volcanic regions and beaches or river gravels.
