igneous
Likely basalt (dark mafic rock)
Basalt; primarily plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray-black, fine-grained and massive, with dull to weakly vitreous luster; hardness about 5–6 Mohs, no obvious cleavage, and typical specific gravity 2.8–3.0. The specimen may be weathered or partially vesicular, though detail is limited by the photograph.
Identified More igneous →
Explore Likely basalt (dark mafic rock) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Dark gray-black, fine-grained and massive, with dull to weakly vitreous luster; hardness about 5–6 Mohs, no obvious cleavage, and typical specific gravity 2.8–3.0. The specimen may be weathered or partially vesicular, though detail is limited by the photograph.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in volcanic flows; most examples are Cenozoic or older, depending on locality. The accompanying speckled rock could be a separate coarse-grained igneous specimen.
Uses & applications
Used extensively as crushed aggregate, road stone, railroad ballast, riprap, and dimension stone; attractive polished basalt is occasionally used decoratively. Ordinary pieces have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. Magnetic basalt may contain magnetite and can weakly attract a magnet.
Field identification & locations
Identify by its dense black-gray color, fine texture, heavy weight, and lack of visible large crystals; a streak test is usually gray, and it does not fizz with dilute hydrochloric acid unless carbonate-filled cavities are present. Common worldwide, especially volcanic regions and oceanic islands.
