igneous
Red-brown rounded cobble, likely iron-rich basalt or andesite
Fine-grained mafic volcanic rock; possible basalt (plagioclase + pyroxene ± olivine, with iron-oxide staining)
AI-generated identification · may be incorrectDark reddish-brown, matte to faintly satin luster, very fine-grained and massive, with no visible cleavage or crystals; estimated Mohs hardness 5–6 and specific gravity about 2.8–3.1. Its smooth, rounded shape indicates water or abrasion transport, and the wet surface deepens the color.
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Physical properties
Dark reddish-brown, matte to faintly satin luster, very fine-grained and massive, with no visible cleavage or crystals; estimated Mohs hardness 5–6 and specific gravity about 2.8–3.1. Its smooth, rounded shape indicates water or abrasion transport, and the wet surface deepens the color.
Formation & geological history
Likely formed when lava cooled rapidly at or near Earth’s surface, then weathered and became rounded in a stream, beach, or glacial deposit. Exact geological age and composition cannot be established from the photograph alone.
Uses & applications
Common basalt is used as crushed aggregate, roadstone, riprap, and construction material; attractive polished pieces may be sold as specimens or decorative stones.
Geological facts
The reddish color commonly results from oxidation of iron-bearing minerals to hematite or limonite. A streak, hardness, density, magnetism, and thin-section or chemical test would distinguish basalt from red sandstone, chert, or ironstone.
Field identification & locations
Look for a dense, fine-grained texture, weak magnetism, and inability to scratch it with a steel knife; it should not fizz with dilute hydrochloric acid. Common in volcanic terrains and river gravels worldwide.
