igneous
Dark mafic rock, likely basalt
Basalt; predominantly plagioclase feldspar, pyroxene, and possible olivine or magnetite
AI-generated identification · may be incorrectUsually fine-grained, dark gray to black, with a dull to faintly vitreous luster; Mohs hardness about 5–6, massive texture, and no prominent cleavage. The specimen appears weathered and may contain rusty iron-oxide staining or small vesicles.
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Physical properties
Usually fine-grained, dark gray to black, with a dull to faintly vitreous luster; Mohs hardness about 5–6, massive texture, and no prominent cleavage. The specimen appears weathered and may contain rusty iron-oxide staining or small vesicles.
Formation & geological history
Basalt forms when low-viscosity mafic lava rapidly cools at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and mid-ocean ridges. Its age cannot be determined from the photograph and may range from recent to billions of years.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, riprap, and building stone; attractive polished pieces are used decoratively. Ordinary hand specimens have little commercial value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. It is commonly denser and darker than rhyolite or granite.
Field identification & locations
Look for a dark, fine-grained, heavy rock that may attract a magnet if magnetite-rich; a hand lens may reveal tiny feldspar crystals or vesicles. Common worldwide, especially Hawaii, Iceland, India’s Deccan Traps, and oceanic terrains.
