igneous
Polished basalt pebble
Basalt; fine-grained mafic volcanic rock, typically plagioclase + pyroxene ± olivine and iron-titanium oxides
AI-generated identification · may be incorrectUsually Mohs 5–6, dark gray to brown-black, with a dull to subtly vitreous polish; fine-grained texture obscures individual crystals, and cleavage is absent. Specific gravity is about 2.8–3.1; the image may show weathering or iron staining and small vesicles.
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Physical properties
Usually Mohs 5–6, dark gray to brown-black, with a dull to subtly vitreous polish; fine-grained texture obscures individual crystals, and cleavage is absent. Specific gravity is about 2.8–3.1; the image may show weathering or iron staining and small vesicles.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes. Rounded shape indicates transport by water or deliberate tumbling/polishing.
Uses & applications
Used as aggregate, roadstone, riprap, paving, landscaping, and decorative stone; polished pieces are sold as inexpensive display stones or lapidary material.
Geological facts
Basalt is Earth’s most abundant volcanic rock and forms much of the oceanic crust. Its dark color reflects abundant ferromagnesian minerals and iron-bearing oxides.
Field identification & locations
Identify by its uniform dark fine-grained appearance, high density, lack of visible large crystals, and inability to scratch with a steel knife easily; acid produces no reaction. Common in volcanic regions and river gravels.
