igneous
Rounded dark pebble, likely basaltic andesite
Fine-grained mafic volcanic rock; probable basalt or andesite, identification uncertain from image alone
AI-generated identification · may be incorrectDark gray-brown with tan mottling, dull to subtly satin luster, and smooth water-worn surface; likely hardness about 5–6 Mohs, massive texture, no visible cleavage, and specific gravity roughly 2.7–3.0.
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Physical properties
Dark gray-brown with tan mottling, dull to subtly satin luster, and smooth water-worn surface; likely hardness about 5–6 Mohs, massive texture, no visible cleavage, and specific gravity roughly 2.7–3.0.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, then was rounded by stream or beach transport; age cannot be determined visually and may range from recent to ancient.
Uses & applications
Commonly used as aggregate, road stone, riprap, landscaping rock, and occasionally as a polished decorative pebble; little collector value unless from a notable locality.
Geological facts
The dark color reflects abundant pyroxene and/or amphibole, with possible feldspar and minor magnetite; brown patches may represent weathering or oxidized iron minerals.
Field identification & locations
Look for a fine-grained, heavy, dark matrix that does not fizz with dilute hydrochloric acid; a streak, magnet test, and fresh broken surface help distinguish basalt from andesite or altered sedimentary rocks. Common worldwide in volcanic regions and river gravels.
