igneous
Dark volcanic rock, likely basalt
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectFine-grained, dark gray to black, dull to slightly vitreous luster; typically massive with no visible cleavage. Mohs hardness about 5–6 and specific gravity about 2.8–3.0; the specimen may also be scoria if its apparent pores are genuine.
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Physical properties
Fine-grained, dark gray to black, dull to slightly vitreous luster; typically massive with no visible cleavage. Mohs hardness about 5–6 and specific gravity about 2.8–3.0; the specimen may also be scoria if its apparent pores are genuine.
Formation & geological history
An extrusive igneous rock formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic eruptions. Exact age cannot be determined from the photograph.
Uses & applications
Used extensively as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive polished basalt is used decoratively. Common pieces have little collector value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify it by its uniform dark color, fine texture, and substantial weight; a streak test is usually gray, and a magnet may show weak attraction if iron minerals are present. Common worldwide in volcanic regions and oceanic settings.
