igneous
Likely weathered basalt
Basalt, a mafic volcanic rock composed mainly of plagioclase feldspar and pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectDark gray, fine- to medium-grained, dull to weakly vitreous luster; typically massive with no cleavage as a whole, Mohs hardness about 5–6, and specific gravity roughly 2.8–3.0. The rough, irregular specimen may show weathering or vesicles, though the image is insufficient for certainty.
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Physical properties
Dark gray, fine- to medium-grained, dull to weakly vitreous luster; typically massive with no cleavage as a whole, Mohs hardness about 5–6, and specific gravity roughly 2.8–3.0. The rough, irregular specimen may show weathering or vesicles, though the image is insufficient for certainty.
Formation & geological history
Basalt forms when low-silica magma rapidly cools at or near Earth’s surface, commonly in lava flows, oceanic crust, and volcanic islands; specimens may range from recent eruptions to Archean rocks over 2.5 billion years old.
Uses & applications
Widely used as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive polished pieces are used decoratively but generally have limited jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Common worldwide, especially Hawaii, Iceland, India’s Deccan Traps, oceanic islands, and mid-ocean ridges; identify by dark color, high density, and fine texture, but confirm with a fresh fracture or thin section because slate, amphibolite, and meteorites can look similar.
