igneous
Likely basalt
Mafic volcanic rock, chiefly plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray-black, fine-grained and massive with a dull to slightly vitreous luster; Mohs hardness about 5–6, no obvious cleavage, and specific gravity roughly 2.8–3.0. The glossy spots may reflect fresh fracture surfaces or minor crystalline minerals.
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Physical properties
Dark gray-black, fine-grained and massive with a dull to slightly vitreous luster; Mohs hardness about 5–6, no obvious cleavage, and specific gravity roughly 2.8–3.0. The glossy spots may reflect fresh fracture surfaces or minor crystalline minerals.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and mid-ocean ridges; specimens may range from recent to billions of years old.
Uses & applications
Widely used as crushed aggregate, road base, railroad ballast, riprap, and occasionally dimension stone; attractive polished pieces are used decoratively but are not usually gemstone grade.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Vesicles, if present, indicate gas bubbles trapped during cooling, while tiny blue-green crystals could represent alteration minerals.
Field identification & locations
Identify it by its uniformly dark color, high density, fine texture, and lack of visible large crystals; a streak, magnetism, and thin-section or chemical test would improve certainty. Common worldwide, especially volcanic regions and oceanic settings.
