igneous
Rounded gray river pebble, likely basalt or andesite
Fine-grained mafic to intermediate volcanic rock; commonly basalt (plagioclase ± pyroxene, possibly olivine)
AI-generated identification · may be incorrectHardness about 5.5–6.5 Mohs; gray-green, dull to slightly waxy luster, fine-grained and massive, with no obvious cleavage. The smooth, rounded surface indicates water abrasion and weathering.
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Physical properties
Hardness about 5.5–6.5 Mohs; gray-green, dull to slightly waxy luster, fine-grained and massive, with no obvious cleavage. The smooth, rounded surface indicates water abrasion and weathering.
Formation & geological history
Likely formed when volcanic lava cooled rapidly at or near Earth’s surface, then was transported and rounded in a stream or beach environment. Its exact geological age and composition cannot be determined from the photograph alone.
Uses & applications
Commonly used as landscaping stone, aggregate, riprap, and decorative pebble; generally has little jewelry value unless unusually attractive or collectible.
Geological facts
Basalt is Earth’s most abundant volcanic rock and forms much of the oceanic crust. Similar-looking stones can also be weathered andesite, greenstone, or fine-grained metamorphic rock.
Field identification & locations
Field clues include high density, fine texture, lack of visible large crystals, and a dark gray to green-gray color; a streak, magnetism test, hardness test, and fresh broken surface would improve identification. Common worldwide, especially in volcanic regions and streambeds.
