igneous
Black river rock, likely basalt
Basalt (fine-grained mafic volcanic rock; identification from photo is tentative)
AI-generated identification · may be incorrectDark blue-black when wet, smooth and rounded by river abrasion, with a glossy wet surface and scattered pale flecks. Basalt is typically composed of plagioclase and pyroxene, often with olivine; hardness about 6 on Mohs scale, generally massive with no visible cleavage.
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Physical properties
Dark blue-black when wet, smooth and rounded by river abrasion, with a glossy wet surface and scattered pale flecks. Basalt is typically composed of plagioclase and pyroxene, often with olivine; hardness about 6 on Mohs scale, generally massive with no visible cleavage.
Formation & geological history
Likely formed when mafic lava cooled at or near the surface, then was transported and rounded in the Skokomish River. Basalt can range widely in age; the image does not establish this specimen’s age or confirm its composition.
Uses & applications
Commonly used as aggregate, road stone, landscaping rock, and occasionally as a polished decorative stone.
Geological facts
A heavy feel and rapid drying are not diagnostic. Pale spots could be mineral grains or surface marks; confirming basalt would require examining a fresh surface or testing mineral composition.
Field identification & locations
Rounded dark rocks occur in river gravels, but similar-looking materials include andesite and other volcanic rocks. Found near the Skokomish River, Washington; value is usually about $0–$10 as a common river specimen, more if verified and attractively prepared.
