igneous
Weathered basalt (likely)
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectDark gray-green, dull to weakly earthy luster, massive and fine-grained; hardness about 5–6 Mohs, no visible cleavage, and typically specific gravity 2.8–3.0. The surface appears weathered, possibly with iron-oxide staining or alteration.
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Physical properties
Dark gray-green, dull to weakly earthy luster, massive and fine-grained; hardness about 5–6 Mohs, no visible cleavage, and typically specific gravity 2.8–3.0. The surface appears weathered, possibly with iron-oxide staining or alteration.
Formation & geological history
Basalt forms when low-silica, iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, flood basalts, and volcanic fields. It may range from Precambrian to modern in age; this specimen’s age cannot be determined visually.
Uses & applications
Used extensively as crushed aggregate, roadstone, riprap, railway ballast, and building stone; attractive polished varieties are used decoratively but are not generally gemstones.
Geological facts
Basalt is the most abundant volcanic rock and forms much of the ocean floor. Vesicles, if present, may later be filled by minerals such as zeolite, calcite, or quartz.
Field identification & locations
Identify it by its dense dark color, fine texture, lack of obvious individual crystals, and resistance to scratching; a steel nail usually does not scratch fresh basalt. Common worldwide, especially volcanic regions and oceanic terrains; laboratory testing is needed to distinguish it from altered andesite or other mafic rocks.
