igneous
Tumbled basalt pebble
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with magnetite
AI-generated identification · may be incorrectTypically dark gray to black, dense, opaque, and fine-grained with a dull to subtly waxy luster; Mohs hardness about 5–6, no visible cleavage, and specific gravity roughly 2.8–3.0. The rounded, smooth surface indicates tumbling or natural abrasion.
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Physical properties
Typically dark gray to black, dense, opaque, and fine-grained with a dull to subtly waxy luster; Mohs hardness about 5–6, no visible cleavage, and specific gravity roughly 2.8–3.0. The rounded, smooth surface indicates tumbling or natural abrasion.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and ancient lava flows. Its age may range from recent eruptions to Precambrian flows; the image alone cannot establish age.
Uses & applications
Used as aggregate, road stone, railway ballast, building stone, landscaping material, and occasionally as a polished decorative pebble or jewelry component.
Geological facts
Basalt is the most abundant volcanic rock on Earth and makes up much of the ocean floor. Its dark color reflects abundant ferromagnesian minerals and iron oxides.
Field identification & locations
Identify it by its black fine-grained appearance, high density, lack of visible crystals, and possible weak magnetic response; it is commonly found in volcanic regions, river gravels, beaches, and landscaping collections.
