igneous
Rounded basalt pebble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and minor olivine or iron oxides
AI-generated identification · may be incorrectDark gray to reddish-brown, dull to subtly vitreous, fine-grained and typically nonfoliated; Mohs hardness about 6, with no visible cleavage and specific gravity around 2.8–3.0. The smooth shape indicates water or sediment abrasion, while reddish areas likely reflect iron-oxide weathering.
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Physical properties
Dark gray to reddish-brown, dull to subtly vitreous, fine-grained and typically nonfoliated; Mohs hardness about 6, with no visible cleavage and specific gravity around 2.8–3.0. The smooth shape indicates water or sediment abrasion, while reddish areas likely reflect iron-oxide weathering.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic plateaus, and shield volcanoes. It may range from recent volcanic material to ancient rocks billions of years old; the specimen’s age cannot be determined visually.
Uses & applications
Used as crushed aggregate, roadstone, railroad ballast, riprap, building stone, and occasionally decorative landscaping material. Ordinary pebbles have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and makes up much of the ocean floor. Its fine texture results from rapid cooling that prevents large crystals from forming.
Field identification & locations
Identify it by its heavy, dark, fine-grained appearance, lack of layering, and inability to scratch easily with a steel knife; a streak or density test can help distinguish it from similar dark stones. Common worldwide, especially along rivers, beaches, and volcanic regions.
