igneous
Dark gray crystalline rock, likely basalt
Basalt; chiefly plagioclase feldspar, clinopyroxene, and commonly olivine or iron-titanium oxides
AI-generated identification · may be incorrectFine- to medium-grained, dark gray to black, with a dull to slightly vitreous luster; typically massive and hard (about Mohs 6), with no obvious cleavage. The specimen appears crystalline and may contain small vesicles or weathered surfaces.
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Physical properties
Fine- to medium-grained, dark gray to black, with a dull to slightly vitreous luster; typically massive and hard (about Mohs 6), with no obvious cleavage. The specimen appears crystalline and may contain small vesicles or weathered surfaces.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and mid-ocean ridges. Its age cannot be determined from the image and may range from recent eruptions to ancient formations.
Uses & applications
Widely used as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive polished varieties are used decoratively. Ordinary hand specimens generally have little commercial or collector value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. It is commonly magnetic when iron-bearing oxides such as magnetite are present.
Field identification & locations
Identify it by its dark color, substantial weight, fine crystalline texture, and lack of visible large quartz or feldspar crystals; a streak test is usually gray, and a magnet may respond weakly. Common localities include Hawaii, Iceland, India’s Deccan Traps, and oceanic crust.
