igneous
Rounded basalt pebble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectDark brown to gray, very fine-grained, dull to weakly submetallic, with no visible crystals or cleavage; Mohs hardness about 6, specific gravity roughly 2.8–3.0. The smooth rounded shape indicates abrasion by water or sediment transport.
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Physical properties
Dark brown to gray, very fine-grained, dull to weakly submetallic, with no visible crystals or cleavage; Mohs hardness about 6, specific gravity roughly 2.8–3.0. The smooth rounded shape indicates abrasion by water or sediment transport.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic fields, or ancient lava flows. Its exact geological age cannot be determined from the photograph.
Uses & applications
Used as aggregate, roadstone, riprap, ballast, and occasionally as a polished decorative stone; this small pebble has little commercial value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Brown coloration may reflect weathering and oxidation of iron-bearing minerals.
Field identification & locations
Identify it by its dense, fine-grained dark appearance, lack of layering, and high density; a streak, fresh broken surface, and magnet test could help confirm it. Common worldwide, especially near volcanic regions and river or beach gravels.
