igneous
Rounded basalt pebble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene
AI-generated identification · may be incorrectDark gray to brown, dull-to-slightly-vitreous luster, dense and fine-grained with minor vesicles or weathering pits; hardness about 5–6 Mohs, no obvious cleavage, specific gravity roughly 2.8–3.0. The curved shape appears naturally water-worn or possibly polished.
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Physical properties
Dark gray to brown, dull-to-slightly-vitreous luster, dense and fine-grained with minor vesicles or weathering pits; hardness about 5–6 Mohs, no obvious cleavage, specific gravity roughly 2.8–3.0. The curved shape appears naturally water-worn or possibly polished.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic provinces and oceanic crust; the specimen’s precise age and source cannot be determined from the photograph.
Uses & applications
Used extensively as crushed aggregate, road stone, riprap, building stone, and landscaping material; attractive polished pieces are sold as curiosities or decorative stones, usually at modest prices.
Geological facts
Basalt is Earth’s most common volcanic rock and commonly forms pillow lavas underwater. Its dark color reflects abundant ferromagnesian minerals, while brown tones may result from surface oxidation.
Field identification & locations
Identify it by its dark, compact, fine-grained texture, heavy feel, and lack of visible large crystals; a streak, magnetism test, and fresh broken surface would improve confidence. Common worldwide, especially Iceland, Hawaii, India, and oceanic volcanic regions.
