igneous
Dark volcanic rock, likely basalt
Basalt; predominantly plagioclase feldspar, pyroxene, and minor olivine or magnetite
AI-generated identification · may be incorrectTypically fine-grained, dark gray to black, with dull to weakly vitreous luster; Mohs hardness about 6, no obvious cleavage, and specific gravity around 2.8–3.0. The specimen appears dense and massive, though image resolution limits certainty.
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Physical properties
Typically fine-grained, dark gray to black, with dull to weakly vitreous luster; Mohs hardness about 6, no obvious cleavage, and specific gravity around 2.8–3.0. The specimen appears dense and massive, though image resolution limits certainty.
Formation & geological history
An extrusive igneous rock formed when mafic lava rapidly cooled at or near Earth’s surface, commonly during the Cenozoic or younger volcanic episodes. Exact age and locality cannot be determined from the image.
Uses & applications
Widely used as crushed aggregate, road base, railroad ballast, riprap, and building stone; attractive polished pieces may be used decoratively but are generally low-value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. Magnetic grains may give it weak magnetism.
Field identification & locations
Identify it by its uniformly dark color, fine texture, substantial weight, and lack of visible large crystals; a streak, magnet, and fresh fractured surface would help distinguish it from slag or dark sedimentary rock. Common worldwide, especially oceanic islands and continental volcanic provinces.
