igneous
Weathered basalt pebble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar and pyroxene, with possible olivine and iron-titanium oxides
AI-generated identification · may be incorrectDark gray, rounded, dull to weakly subvitreous luster; typically massive and fine-grained, with small vesicles or weathering pits. Hardness about 5–6 Mohs; no obvious cleavage; specific gravity roughly 2.8–3.0.
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Physical properties
Dark gray, rounded, dull to weakly subvitreous luster; typically massive and fine-grained, with small vesicles or weathering pits. Hardness about 5–6 Mohs; no obvious cleavage; specific gravity roughly 2.8–3.0.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, commonly in oceanic crust, lava flows, or volcanic islands; basalt ranges from ancient Precambrian rocks to modern eruptions. The rounded shape and surface pits indicate stream or beach abrasion and weathering.
Uses & applications
Used as crushed aggregate, road base, concrete material, railroad ballast, riprap, and occasionally dimension stone; ordinary pebbles have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant ferromagnesian minerals and iron-rich oxides.
Field identification & locations
Identify it by its dark, dense, fine-grained texture, lack of visible large crystals, and possible vesicles; a streak, magnetic response, and acid test can help distinguish it from similar dark rocks. Common worldwide, especially volcanic regions and river or beach gravels.
