igneous
Rounded dark speckled pebble, likely basalt
Basalt; fine-grained mafic volcanic rock, typically composed of plagioclase, clinopyroxene, and iron-magnesium minerals
AI-generated identification · may be incorrectDark gray to black, fine-grained, matte to slightly subvitreous, with tiny pale mineral specks; hardness about 5–6 Mohs, massive texture, no visible cleavage, and specific gravity roughly 2.8–3.0. Its smooth shape indicates water or sediment abrasion, while the surface may be naturally weathered or tumbled.
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Physical properties
Dark gray to black, fine-grained, matte to slightly subvitreous, with tiny pale mineral specks; hardness about 5–6 Mohs, massive texture, no visible cleavage, and specific gravity roughly 2.8–3.0. Its smooth shape indicates water or sediment abrasion, while the surface may be naturally weathered or tumbled.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and flood-basalt provinces. Its exact geological age cannot be determined from the image; basalt ranges from recent to Precambrian.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, riprap, and occasionally dimension stone or decorative landscaping rock. Ordinary pebbles have negligible jewelry or collector value.
Geological facts
The pale flecks may be plagioclase phenocrysts or weathering minerals; a magnet may show weak attraction if the rock contains magnetite. The image does not support a more specific identification such as gabbro, diabase, or meteorite.
Field identification & locations
Common in volcanic regions, river gravels, beaches, and glacial deposits. Confirm with a fresh broken surface, streak test, magnetism, and texture; basalt is typically opaque and lacks metallic meteorite-like density or fusion crust.
