igneous
Weathered basalt pebble
Basalt, a fine-grained mafic volcanic rock; likely olivine-bearing basalt
AI-generated identification · may be incorrectDark gray to olive-green, dull to slightly earthy luster, very fine-grained and massive; the green areas may be altered olivine or chlorite, with possible tiny vesicles. Hardness is typically about 5–6 Mohs; cleavage is absent and specific gravity is roughly 2.8–3.1.
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Physical properties
Dark gray to olive-green, dull to slightly earthy luster, very fine-grained and massive; the green areas may be altered olivine or chlorite, with possible tiny vesicles. Hardness is typically about 5–6 Mohs; cleavage is absent and specific gravity is roughly 2.8–3.1.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly in volcanic fields or oceanic crust. Basalt ranges from recent volcanic rocks to Archean examples older than 2.5 billion years; the rounded shape indicates stream or beach transport and weathering.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, paving stone, and occasionally dimension stone. Ordinary pebbles have little jewelry value.
Geological facts
The olive coloration can result from weathered olivine, serpentinization, or other secondary alteration. Its compact texture and dark mafic patches are typical of basalt rather than granite.
Field identification & locations
Identify it by its heavy feel, dark fine-grained texture, lack of visible quartz, and weak or absent reaction to dilute acid; a streak test is usually gray. Common worldwide, especially volcanic islands, ocean margins, and basalt plateaus.
