igneous
Dark basalt (likely fine-grained mafic rock)
Basalt; primarily plagioclase feldspar and pyroxene, with possible olivine and magnetite
AI-generated identification · may be incorrectTypically Mohs 5–6, dark gray to black, dull to slightly vitreous luster, very fine-grained with interlocking crystals; cleavage is generally absent, and specific gravity is about 2.8–3.0. The specimen appears dense and massive, though its exact identity cannot be confirmed from a photograph alone.
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Physical properties
Typically Mohs 5–6, dark gray to black, dull to slightly vitreous luster, very fine-grained with interlocking crystals; cleavage is generally absent, and specific gravity is about 2.8–3.0. The specimen appears dense and massive, though its exact identity cannot be confirmed from a photograph alone.
Formation & geological history
Basalt forms when low-silica magma erupts and cools rapidly at or near Earth’s surface, commonly as lava flows, dikes, or oceanic crust. It may range from Precambrian to modern in age and is widespread in volcanic terrains.
Uses & applications
Used as crushed aggregate, road stone, railway ballast, riprap, and building stone; attractive polished varieties may be used decoratively. Ordinary hand specimens generally have little commercial or collector value.
Geological facts
Its dark color reflects abundant iron- and magnesium-rich minerals. Vesicles, if present, may later fill with zeolites, calcite, or quartz, producing amygdaloidal basalt.
Field identification & locations
Look for a heavy, dark, fine-grained rock that commonly attracts a weak magnet if magnetite-rich; basalt lacks the visible quartz grains typical of granite. Common locations include Hawaii, Iceland, oceanic crust, and continental volcanic provinces.
