igneous
Dark gray fine-grained stone, likely basalt
Basalt; primarily plagioclase feldspar and pyroxene, commonly with minor olivine and iron oxides
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, dull to slightly subvitreous, with interlocking microscopic crystals; Mohs hardness about 6, no obvious cleavage, and specific gravity roughly 2.8–3.0. The specimen appears irregular and weathered, but its exact identification cannot be confirmed from the image alone.
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Physical properties
Typically dark gray to black, fine-grained, dull to slightly subvitreous, with interlocking microscopic crystals; Mohs hardness about 6, no obvious cleavage, and specific gravity roughly 2.8–3.0. The specimen appears irregular and weathered, but its exact identification cannot be confirmed from the image alone.
Formation & geological history
Basalt forms when low-silica mafic lava rapidly cools at or near Earth’s surface, commonly in volcanic flows and oceanic crust. Most examples are geologically young relative to continental rocks, ranging from recent eruptions to Precambrian formations.
Uses & applications
Used extensively as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive polished pieces may be sold as decorative stone or collected. It has little gemstone value unless it contains distinctive vesicles, zeolite crystals, or other inclusions.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. It commonly has a higher density and darker color than granite.
Field identification & locations
Identify it by its dark color, heavy feel, fine texture, and lack of visible large crystals; a streak is usually gray, and it does not fizz with dilute acid. Common localities include volcanic regions, oceanic islands, mid-ocean ridges, and large lava plateaus.
