igneous
Tumbled iron-rich volcanic rock, likely basalt
Basalt; predominantly plagioclase and pyroxene with iron-oxide alteration
AI-generated identification · may be incorrectDark brown to black, rusty weathering, fine-grained and opaque with a dull to weakly vitreous luster; Mohs hardness about 5–6, negligible cleavage, and specific gravity roughly 2.8–3.1. The rounded shape appears naturally water-worn or tumbled.
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Physical properties
Dark brown to black, rusty weathering, fine-grained and opaque with a dull to weakly vitreous luster; Mohs hardness about 5–6, negligible cleavage, and specific gravity roughly 2.8–3.1. The rounded shape appears naturally water-worn or tumbled.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly in oceanic crust and volcanic fields; its exact age cannot be determined from the photograph. Brown coloration likely reflects oxidation of iron-bearing minerals during weathering.
Uses & applications
Used as aggregate, roadstone, riprap, landscaping stone, and occasionally as a polished decorative or lapidary material.
Geological facts
Basalt is the most abundant volcanic rock and commonly contains microscopic crystals rather than visible grains. Rusty surfaces can result from limonite or hematite coatings.
Field identification & locations
Identify it by its dense, fine texture, dark mafic color, lack of obvious layering, and possible attraction to a strong magnet if iron-rich; a streak, density, and thin-section test would confirm it. Common worldwide, especially volcanic regions and stream gravels.
