igneous
Tumbled basalt
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectTypically dark gray to black, here weathered brown with a subdued vitreous-to-dull luster and abundant vesicles (gas cavities). Hardness is about 5–6 Mohs; opaque, massive, and lacking visible cleavage in the rock aggregate.
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Physical properties
Typically dark gray to black, here weathered brown with a subdued vitreous-to-dull luster and abundant vesicles (gas cavities). Hardness is about 5–6 Mohs; opaque, massive, and lacking visible cleavage in the rock aggregate.
Formation & geological history
Forms when basaltic lava rapidly cools at or near Earth’s surface, commonly in volcanic flows; vesicles record trapped gas. Basalt ranges from Precambrian to modern age, and this specimen appears naturally rounded or tumbled.
Uses & applications
Used as crushed aggregate, roadstone, building stone, and occasionally as decorative tumbled stone or jewelry material. Common specimens generally have modest collector value.
Geological facts
The hole-rich texture is called vesicular basalt; cavities may later become lined with minerals such as zeolite, calcite, or quartz. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify it by its dark fine-grained matrix, heavy feel, irregular vesicles, and lack of large crystals; a steel file usually scratches it only with difficulty. Common worldwide near volcanic regions, including Hawaii, Iceland, India, and the Pacific Northwest.
