igneous
Polished basalt pebble
Basalt, a fine-grained mafic volcanic rock dominated by plagioclase and pyroxene, commonly with olivine and iron-titanium oxides
AI-generated identification · may be incorrectDark gray-black, fine-grained, opaque, and typically dull to weakly vitreous; hardness about 5–6 Mohs, no prominent cleavage, and specific gravity roughly 2.8–3.0. The reddish metallic flecks may be iron oxides or exposed mineral grains; the glossy surface is from polishing.
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Physical properties
Dark gray-black, fine-grained, opaque, and typically dull to weakly vitreous; hardness about 5–6 Mohs, no prominent cleavage, and specific gravity roughly 2.8–3.0. The reddish metallic flecks may be iron oxides or exposed mineral grains; the glossy surface is from polishing.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and flood-basalt provinces. Individual pieces may be geologically young or billions of years old depending on source.
Uses & applications
Used extensively as crushed aggregate, road base, riprap, dimension stone, and occasionally decorative polished stone; it has little gemstone value unless unusually patterned or vesicular.
Geological facts
Basalt is the most common volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant ferromagnesian minerals and iron-bearing oxides.
Field identification & locations
Identify it by its uniformly dark, dense appearance and fine texture; a steel point generally scratches it only weakly, while dilute acid produces no reaction. Common worldwide, especially volcanic regions; the image alone cannot confirm whether it might instead be a dark iron-rich stone or slag.
