igneous
Rounded gray river pebble, likely basalt or andesite
Fine-grained mafic volcanic rock; commonly basalt (plagioclase, pyroxene, ± olivine)
AI-generated identification · may be incorrectDark gray, fine-grained, massive, and weakly granular with a dull to faintly silky luster; estimated Mohs hardness 5–6, no visible cleavage, and specific gravity about 2.8–3.1. The smooth oval shape reflects water abrasion rather than crystal structure.
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Physical properties
Dark gray, fine-grained, massive, and weakly granular with a dull to faintly silky luster; estimated Mohs hardness 5–6, no visible cleavage, and specific gravity about 2.8–3.1. The smooth oval shape reflects water abrasion rather than crystal structure.
Formation & geological history
Formed when lava cooled rapidly at or near Earth’s surface, then was transported and rounded in a stream or beach environment. The exact geological age cannot be determined from the image and may range from recent to ancient.
Uses & applications
Commonly used as landscaping stone, drainage aggregate, construction aggregate, and decorative gravel; generally little jewelry or collector value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and makes up much of the oceanic crust. Andesite can look similar but is typically slightly lighter and richer in plagioclase.
Field identification & locations
Identify by its uniform fine texture, gray-black color, heft, and lack of visible large crystals; a streak, magnetism test, and thin-section analysis would improve identification. Common worldwide, especially in volcanic regions and riverbeds.
