igneous
Tumbled basalt
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine and magnetite
AI-generated identification · may be incorrectTypically black to dark gray, fine-grained, opaque, and subvitreous to dull; hardness about 5–6 Mohs, massive texture, and high density around 2.8–3.0 g/cm³. The rounded shape and sheen indicate mechanical tumbling/polishing; small pale grains may be feldspar.
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Physical properties
Typically black to dark gray, fine-grained, opaque, and subvitreous to dull; hardness about 5–6 Mohs, massive texture, and high density around 2.8–3.0 g/cm³. The rounded shape and sheen indicate mechanical tumbling/polishing; small pale grains may be feldspar.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes. It occurs in oceanic crust, lava flows, volcanic islands, and flood-basalt provinces.
Uses & applications
Used as aggregate, roadstone, construction material, railway ballast, and basalt fiber; polished pieces are sold as inexpensive décor, worry stones, and teaching specimens.
Geological facts
Basalt is the most common volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant ferromagnesian minerals and iron oxides.
Field identification & locations
Identify it by its uniformly dark, dense, fine-grained appearance, lack of visible large crystals, and possible weak magnetism from magnetite; distinguish it from glass by its granular texture and natural mineral specks. Common sources include Hawaii, Iceland, India, the Columbia River region, and ocean beaches.
