igneous
Weathered basalt (probable)
Basalt; chiefly plagioclase feldspar and pyroxene, often with olivine and iron oxides
AI-generated identification · may be incorrectFine-grained, massive, dull gray-brown to olive from weathering, with little visible crystal structure; typically Mohs 5–6, nonmetallic luster, negligible cleavage, and specific gravity about 2.8–3.0. The rounded edges suggest water or surface abrasion.
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Physical properties
Fine-grained, massive, dull gray-brown to olive from weathering, with little visible crystal structure; typically Mohs 5–6, nonmetallic luster, negligible cleavage, and specific gravity about 2.8–3.0. The rounded edges suggest water or surface abrasion.
Formation & geological history
Basalt forms when low-silica mafic lava rapidly cools at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and ancient lava flows. Its exact age cannot be determined from the photograph; brown coloration likely reflects oxidation and weathering.
Uses & applications
Used as crushed aggregate, roadstone, railroad ballast, riprap, and building stone; attractive polished pieces may be used decoratively. This ordinary specimen has little gem or collector value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Vesicles, if present, may later fill with minerals such as zeolites, calcite, or quartz.
Field identification & locations
Identify it by its heavy, fine-grained, dark gray-to-brown appearance and lack of obvious layering or large crystals; a streak, magnetism test, and fresh broken surface would improve confidence. Common worldwide, especially near volcanic regions.
