igneous
Basalt
Mafic volcanic rock, chiefly plagioclase feldspar, pyroxene, and commonly olivine or magnetite
AI-generated identification · may be incorrectDark gray to black, fine-grained and massive, with a dull to weakly vitreous luster; hardness about 5–6 Mohs, no visible cleavage, and specific gravity roughly 2.8–3.0. The bright flecks may be feldspar, pyroxene, or weathered mineral surfaces.
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Physical properties
Dark gray to black, fine-grained and massive, with a dull to weakly vitreous luster; hardness about 5–6 Mohs, no visible cleavage, and specific gravity roughly 2.8–3.0. The bright flecks may be feldspar, pyroxene, or weathered mineral surfaces.
Formation & geological history
Forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and large igneous provinces. The specimen’s exact age and locality cannot be determined from the photograph.
Uses & applications
Widely used as crushed aggregate, road metal, railroad ballast, riprap, and building stone; attractive pieces are collected but are generally not jewelry gemstones.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Vesicles, if present, may later fill with zeolites, calcite, or quartz.
Field identification & locations
Identify it by its black, dense, fine-grained appearance and lack of obvious large crystals; a streak is typically gray-black, and it is not strongly attracted to a magnet unless magnetite-rich. Common localities include Hawaii, Iceland, India’s Deccan Traps, and oceanic volcanic regions.
