igneous
Dark mafic volcanic rock, likely basalt
Basalt; chiefly plagioclase feldspar, pyroxene, and minor olivine/iron oxides
AI-generated identification · may be incorrectFine- to medium-grained, dark gray to brown, mostly dull to slightly vitreous; typically hardness 5–6, no obvious cleavage, dense (specific gravity about 2.8–3.0). Orange-brown spots may be weathered olivine or iron-oxide staining; the broken surfaces appear unpolished.
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Physical properties
Fine- to medium-grained, dark gray to brown, mostly dull to slightly vitreous; typically hardness 5–6, no obvious cleavage, dense (specific gravity about 2.8–3.0). Orange-brown spots may be weathered olivine or iron-oxide staining; the broken surfaces appear unpolished.
Formation & geological history
Forms when low-silica, iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic flows, and dikes. Basalt ranges from ancient Precambrian rocks to modern eruptions.
Uses & applications
Used widely as crushed aggregate, road base, riprap, railroad ballast, and building stone; attractive fine-grained pieces may be collected, but this specimen appears common and unremarkable.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. It can be magnetic if it contains sufficient magnetite.
Field identification & locations
Identify by its dark color, heavy weight, fine texture, and lack of visible large crystals; a streak test is generally gray, and a magnet may show attraction. Common worldwide, especially Hawaii, Iceland, India’s Deccan Traps, and oceanic volcanic regions.
