igneous
Rounded gray river pebble, likely basalt or andesite
Fine-grained mafic to intermediate volcanic rock; commonly basalt (plagioclase ± pyroxene/olivine)
AI-generated identification · may be incorrectDark gray, fine-grained, dull to weakly satin luster, with rounded water-worn surfaces and a natural fracture or vein. Typical hardness is about 5–6 Mohs; cleavage is absent and specific gravity roughly 2.7–3.1.
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Physical properties
Dark gray, fine-grained, dull to weakly satin luster, with rounded water-worn surfaces and a natural fracture or vein. Typical hardness is about 5–6 Mohs; cleavage is absent and specific gravity roughly 2.7–3.1.
Formation & geological history
Formed when lava cooled rapidly at or near Earth’s surface, then was rounded by stream or beach abrasion; the exact geological age cannot be determined from the image. The pale specks and crack may represent feldspar, weathering, or a mineral-filled fracture.
Uses & applications
Common basalt is used as aggregate, riprap, paving stone, and occasionally ornamental material; this small specimen has mainly educational or modest collecting value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and forms much of the oceanic crust. Andesite is a similar volcanic rock but generally contains more silica and lighter-colored minerals.
Field identification & locations
Look for a uniformly fine texture, heavy feel, gray-black color, and lack of visible large crystals; a streak, acid, density, and magnification test would improve identification. Common sources include river gravels and volcanic regions worldwide.
