igneous
Rounded basalt cobble
Basalt; primarily plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectFine-grained, dark brown to gray, dull to subtly vitreous luster, with a massive texture and no visible cleavage; typically Mohs 5–6 and specific gravity about 2.8–3.0. The smooth shape indicates water or abrasion, while the surface may be weathered or lightly polished.
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Physical properties
Fine-grained, dark brown to gray, dull to subtly vitreous luster, with a massive texture and no visible cleavage; typically Mohs 5–6 and specific gravity about 2.8–3.0. The smooth shape indicates water or abrasion, while the surface may be weathered or lightly polished.
Formation & geological history
An extrusive mafic igneous rock formed when low-silica lava cooled rapidly at or near Earth’s surface, generally in geologically young volcanic terrains but also in ancient flood-basalt provinces. This specimen was likely rounded in a river, beach, or glacial deposit.
Uses & applications
Used extensively as crushed aggregate, road stone, riprap, railroad ballast, and building stone; attractive dense pieces may be used as garden or display stones. Common specimens usually have little commercial collector value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and forms much of the seafloor. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify by its uniform fine grain, high density, dark color, and lack of obvious layering or large crystals; a streak, magnetism test, and hardness test can help distinguish it from andesite or weathered slag. Common worldwide, especially volcanic islands, ocean beaches, rivers, and glacial gravels.
