igneous
Weathered basaltic rock
Basalt, likely altered basalt (plagioclase–pyroxene ± olivine; possible iron-oxide staining)
AI-generated identification · may be incorrectDark gray, fine- to medium-grained, dull to weakly lustered and massive; hardness about 5–6 Mohs, with no obvious cleavage. Yellow-brown patches are likely limonite/goethite weathering products.
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Physical properties
Dark gray, fine- to medium-grained, dull to weakly lustered and massive; hardness about 5–6 Mohs, with no obvious cleavage. Yellow-brown patches are likely limonite/goethite weathering products.
Formation & geological history
An extrusive igneous rock formed when basaltic lava rapidly cooled at or near Earth’s surface; its exact age cannot be determined from the photograph. Surface oxidation and alteration produced the rusty staining.
Uses & applications
Common basalt is used as crushed aggregate, road base, riprap, concrete stone, and occasionally dimension stone. This ordinary specimen has little jewelry or collector value.
Geological facts
Basalt commonly contains microscopic crystals because of rapid cooling, although weathering can obscure its texture. Magnetic response may be present if magnetite is abundant.
Field identification & locations
Identify it by its dense dark-gray color, fine texture, massive form, and lack of visible sedimentary layering; a steel scratch and weak magnetism can support the identification. Common worldwide, especially in volcanic regions and oceanic crust.
