igneous
Weathered basalt with iron-oxide alteration
Basalt, principally plagioclase and pyroxene, with secondary iron oxides/hydroxides
AI-generated identification · may be incorrectDark gray-black, fine-grained, massive volcanic rock with dull to slightly waxy luster and rusty yellow-brown surface staining; hardness about 5–6, no obvious cleavage, typical specific gravity 2.8–3.1. The rounded shape indicates stream or surface weathering.
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Physical properties
Dark gray-black, fine-grained, massive volcanic rock with dull to slightly waxy luster and rusty yellow-brown surface staining; hardness about 5–6, no obvious cleavage, typical specific gravity 2.8–3.1. The rounded shape indicates stream or surface weathering.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, commonly in volcanic flows, dikes, or oceanic crust; its precise age cannot be determined from the image. Brown-gold patches likely represent weathering of iron-bearing minerals to limonite/goethite.
Uses & applications
Common basalt is used as crushed aggregate, road metal, riprap, and occasionally dimension stone; this small specimen has mainly educational or modest collecting value.
Geological facts
Basalt is the most abundant volcanic rock and forms much of the ocean floor. The mottled golden areas are alteration coatings rather than visible gold.
Field identification & locations
Identify it by its heavy, fine-grained dark body, lack of visible large crystals, and strong attraction only weakly or variably to a magnet; a streak, fresh broken surface, and acid test help distinguish it from slag or other dark rocks. Common worldwide, especially volcanic regions and streambeds.
