igneous
Basalt pebble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray to black, dull to faintly lusterous, fine-grained and typically massive; hardness about 5–6 Mohs, with no visible cleavage and specific gravity roughly 2.8–3.0. The smooth, flattened shape indicates water abrasion or deliberate tumbling/polishing.
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Physical properties
Dark gray to black, dull to faintly lusterous, fine-grained and typically massive; hardness about 5–6 Mohs, with no visible cleavage and specific gravity roughly 2.8–3.0. The smooth, flattened shape indicates water abrasion or deliberate tumbling/polishing.
Formation & geological history
Forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic fields and oceanic crust. Basalt ranges from ancient Precambrian flows to modern eruptions; the specimen’s age cannot be determined from appearance.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, concrete aggregate, dimension stone, and occasionally as a decorative or lapidary material.
Geological facts
Basalt is Earth’s most abundant volcanic rock and makes up much of the ocean floor. Vesicles may occur where gas bubbles were trapped, though this specimen appears dense and nonvesicular.
Field identification & locations
Identify it by its uniform dark color, fine texture, heavy feel, and lack of obvious crystals; distinguish it from slate by its massive texture and from dark glass by its granular surface. Common worldwide, especially near volcanic regions, riverbeds, and beaches.
