igneous
Basalt
Basalt; predominantly plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to slightly vitreous luster, typically massive with no obvious cleavage; Mohs hardness about 5–6 and specific gravity approximately 2.8–3.0. The specimen appears fractured and may contain small vesicles or weathered cavities.
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Physical properties
Dark gray to black, fine-grained, dull to slightly vitreous luster, typically massive with no obvious cleavage; Mohs hardness about 5–6 and specific gravity approximately 2.8–3.0. The specimen appears fractured and may contain small vesicles or weathered cavities.
Formation & geological history
An extrusive igneous rock formed when mafic lava rapidly cooled at or near Earth’s surface, commonly during volcanic eruptions; specimens may range from ancient Precambrian flows to recent lava. The visible piece is consistent with ordinary volcanic basalt, though an image cannot establish age precisely.
Uses & applications
Widely used as crushed aggregate, road base, railroad ballast, building stone, and occasionally in landscaping or sculpture; fresh attractive pieces are collected but usually have modest value.
Geological facts
Basalt makes up much of the oceanic crust and forms extensive lava plateaus and volcanic islands. It commonly occurs with vesicular textures, columnar jointing, or secondary minerals such as zeolites and calcite.
Field identification & locations
Identify it by its uniformly dark color, heavy feel, fine texture, and lack of visible large crystals; a streak test is usually gray, and it is not strongly attracted to a magnet. Common localities include oceanic crust, Hawaii, Iceland, India’s Deccan Traps, and flood-basalt provinces worldwide.
