igneous
Polished basalt pebble
Basalt; predominantly plagioclase feldspar, pyroxene, and minor olivine/iron oxides
AI-generated identification · may be incorrectDark gray-black, fine-grained, massive, and subvitreous to dull; hardness about 5–6 Mohs, no visible cleavage, typical specific gravity 2.8–3.0. The rounded shape and glossy surface indicate water wear or polishing; rusty-brown areas are likely iron-oxide weathering.
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Physical properties
Dark gray-black, fine-grained, massive, and subvitreous to dull; hardness about 5–6 Mohs, no visible cleavage, typical specific gravity 2.8–3.0. The rounded shape and glossy surface indicate water wear or polishing; rusty-brown areas are likely iron-oxide weathering.
Formation & geological history
An extrusive mafic igneous rock formed when basaltic lava cooled rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and flood-basalt provinces. Its exact geological age cannot be determined from the image.
Uses & applications
Used as aggregate, riprap, paving stone, landscaping material, and decorative polished stone; generally low commercial value as a specimen.
Geological facts
Basalt is the most widespread volcanic rock and forms much of Earth’s oceanic crust. It commonly contains microscopic crystals because of rapid cooling.
Field identification & locations
Identify by its uniformly dark, fine texture, high density, and lack of large crystals or layering; a streak, fresh fracture, and magnet test can help distinguish it from other dark rocks. Common worldwide, especially river gravels and volcanic regions.
