igneous
Black basalt pebble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and often olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to weakly vitreous luster; typically hardness 5–6 Mohs, massive texture, no obvious cleavage, and specific gravity about 2.8–3.0. The rounded shape indicates water or abrasion wear, and small pale grains may be feldspar or alteration minerals.
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Physical properties
Dark gray to black, fine-grained, dull to weakly vitreous luster; typically hardness 5–6 Mohs, massive texture, no obvious cleavage, and specific gravity about 2.8–3.0. The rounded shape indicates water or abrasion wear, and small pale grains may be feldspar or alteration minerals.
Formation & geological history
Forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, lava flows, and volcanic islands. Individual basalt formation ages range from ancient Archean rocks to modern eruptions.
Uses & applications
Used as crushed aggregate, road base, railway ballast, riprap, and dimension stone; attractive polished pieces are used decoratively but usually have low jewelry value.
Geological facts
Basalt is the most widespread volcanic rock and makes up much of the ocean floor. It commonly has magnetic properties because of disseminated magnetite.
Field identification & locations
Identify by its uniformly dark color, very fine texture, heavy feel, and lack of visible large crystals; it generally does not fizz with dilute acid. Common worldwide, especially volcanic regions and river or beach gravels.
