igneous
Tumbled basalt
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectHardness about 6 Mohs; dark gray to black with brown weathering, dull to subtly vitreous luster, and very fine interlocking crystals. Massive, with no obvious cleavage; specific gravity typically 2.8–3.0.
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Physical properties
Hardness about 6 Mohs; dark gray to black with brown weathering, dull to subtly vitreous luster, and very fine interlocking crystals. Massive, with no obvious cleavage; specific gravity typically 2.8–3.0.
Formation & geological history
Forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. Most specimens are geologically young relative to Earth history, ranging from recent lava to Precambrian flows.
Uses & applications
Used as crushed aggregate, road stone, riprap, building stone, and occasionally for polished decorative objects. This small rounded piece is mainly a display or teaching specimen.
Geological facts
The dark color reflects abundant ferromagnesian minerals and little quartz; weathering can produce brown or reddish surfaces. Its smooth shape indicates natural abrasion or tumbling rather than crystal growth.
Field identification & locations
Identify it by its uniformly dark, dense appearance, fine texture, lack of visible large crystals, and inability to scratch it easily with a steel knife. Common worldwide, especially near volcanic regions; estimated value is about $2–$15 for a small common specimen.
