igneous
Rounded dark volcanic pebble
Likely basalt or basaltic andesite
AI-generated identification · may be incorrectFine-grained, dark gray-green, dull to weakly waxy luster, massive and well-rounded by abrasion; likely Mohs 5–6, with no visible cleavage. The pale streak may be a mineral vein or surface scratch; specific gravity commonly ~2.8–3.0.
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Physical properties
Fine-grained, dark gray-green, dull to weakly waxy luster, massive and well-rounded by abrasion; likely Mohs 5–6, with no visible cleavage. The pale streak may be a mineral vein or surface scratch; specific gravity commonly ~2.8–3.0.
Formation & geological history
An extrusive volcanic rock formed when basaltic lava cooled rapidly, then became rounded in a stream, beach, or gravel deposit. Basaltic rocks are common in the Monterey Bay–central California region, though the exact age cannot be determined from the photograph.
Uses & applications
Used as aggregate, road base, riprap, landscaping stone, and occasionally as a decorative or lapidary material; this ordinary pebble has little commercial value.
Geological facts
Basalt commonly contains plagioclase and pyroxene, with possible olivine or magnetite. Its dark color and fine texture distinguish it from lighter granitic pebbles.
Field identification & locations
Likely local alluvial or beach gravel near 36.9°N, 121.4°W. Confirm with a fresh break: basalt is hard, non-foliated, generally nonporous, and may weakly attract a magnet if magnetite-rich; a hand lens may reveal tiny crystals.
