igneous
Rounded weathered cobble, likely basaltic/volcanic rock
Fine-grained mafic volcanic rock, probably basalt (exact identification requires testing)
AI-generated identification · may be incorrectDark gray-blue, fine-grained, dull to weakly waxy luster, with rusty-brown iron-oxide staining and possible vesicles or weathering pits. Basalt typically has Mohs hardness about 6, no visible cleavage, interlocking microscopic crystals, and specific gravity around 2.8–3.0.
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Physical properties
Dark gray-blue, fine-grained, dull to weakly waxy luster, with rusty-brown iron-oxide staining and possible vesicles or weathering pits. Basalt typically has Mohs hardness about 6, no visible cleavage, interlocking microscopic crystals, and specific gravity around 2.8–3.0.
Formation & geological history
Likely formed when mafic lava cooled rapidly at or near Earth’s surface, then was rounded by stream or shoreline transport and weathered; its exact age and source cannot be determined from the photograph.
Uses & applications
Common basalt is used as crushed aggregate, roadstone, riprap, and building stone; attractive polished pieces may be collected but usually have little commercial value.
Geological facts
The smooth oval shape is transport-worn rather than a crystal habit. Brown patches commonly represent oxidation of iron-bearing minerals such as pyroxene or magnetite.
Field identification & locations
Look for a uniformly fine-grained dark interior, substantial heft, and lack of fizz with dilute hydrochloric acid; a streak, magnet, and fresh broken surface help distinguish basalt from chert, slag, or altered sedimentary rocks. Common in volcanic regions, riverbeds, and beaches.
