igneous
Tumbled basalt pebble
Basalt, a fine-grained mafic volcanic rock; commonly plagioclase, pyroxene, and minor olivine or magnetite
AI-generated identification · may be incorrectDark brown-black, fine-grained, and rounded by natural abrasion or tumbling, with dull to subtly vitreous luster and scattered tiny vesicles. Hardness is roughly 5–6 Mohs; cleavage is generally absent as a rock, and specific gravity is about 2.8–3.0.
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Physical properties
Dark brown-black, fine-grained, and rounded by natural abrasion or tumbling, with dull to subtly vitreous luster and scattered tiny vesicles. Hardness is roughly 5–6 Mohs; cleavage is generally absent as a rock, and specific gravity is about 2.8–3.0.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic activity; the smooth shape indicates water transport or artificial tumbling.
Uses & applications
Used as aggregate, road stone, riprap, landscaping material, and occasionally as a polished or tumbled decorative stone. This specimen has modest collectible value.
Geological facts
Basalt is the most common volcanic rock on oceanic crust and forms much of the seafloor. Its dark color reflects abundant ferromagnesian minerals and low silica content.
Field identification & locations
Identify it by its heavy, dark, fine-grained texture, lack of visible large crystals, and possible vesicles; a streak, magnetism test, and fresh surface help distinguish it from darker sedimentary rocks. Common worldwide, especially volcanic regions and river beaches.
