igneous
Dark gray igneous rock, likely basalt
Basalt; fine-grained plagioclase–pyroxene rock, commonly with minor olivine and magnetite
AI-generated identification · may be incorrectHardness about 6 on the Mohs scale; dark gray to black, dull to slightly subvitreous luster, very fine-grained interlocking crystals, and no obvious cleavage. The pieces show fresh, angular fracture surfaces and a dense, massive texture.
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Physical properties
Hardness about 6 on the Mohs scale; dark gray to black, dull to slightly subvitreous luster, very fine-grained interlocking crystals, and no obvious cleavage. The pieces show fresh, angular fracture surfaces and a dense, massive texture.
Formation & geological history
Basalt forms when low-viscosity mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and flood-basalt provinces. Individual specimens may range from recent eruptions to billions of years old; age cannot be determined from the image.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, riprap, building stone, and basalt fiber; attractive polished varieties are used decoratively. Common pieces have little jewelry or collector value.
Geological facts
Its dark color reflects abundant iron- and magnesium-rich minerals. Vesicles, if present, indicate trapped volcanic gas, while tiny pale crystals are commonly plagioclase feldspar.
Field identification & locations
Common worldwide, especially in Hawaii, Iceland, India’s Deccan Traps, oceanic crust, and volcanic regions. Confirm with a fresh surface, dense fine grain, lack of acid reaction, and occasional magnetic response; it could also be a closely related diabase or andesite.
