igneous
Dark gray-green fine-grained rock, likely basalt
Basalt; chiefly plagioclase feldspar and pyroxene, commonly with olivine or iron oxides
AI-generated identification · may be incorrectTypically dark gray to greenish-black, fine-grained, dull to faintly vitreous, with microscopic crystals; Mohs hardness about 6, no obvious cleavage, and specific gravity roughly 2.8–3.0. The specimen appears dense and massive, though its weathered surface makes exact identification uncertain.
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Physical properties
Typically dark gray to greenish-black, fine-grained, dull to faintly vitreous, with microscopic crystals; Mohs hardness about 6, no obvious cleavage, and specific gravity roughly 2.8–3.0. The specimen appears dense and massive, though its weathered surface makes exact identification uncertain.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic flows and oceanic crust. It may range from Precambrian to modern age; this photograph alone cannot establish its age or locality.
Uses & applications
Used extensively as crushed aggregate, roadstone, railway ballast, riprap, and construction stone; attractive polished varieties are used decoratively. Ordinary hand specimens generally have modest collector value.
Geological facts
Basalt is the most abundant volcanic rock and forms much of the seafloor and many shield volcanoes. It can resemble fine-grained diabase, and laboratory texture or thin-section study may be needed for confirmation.
Field identification & locations
Look for a heavy, uniformly fine-grained dark rock lacking visible sedimentary layers; a streak is usually gray, and a magnet may show weak attraction if iron-rich. Common worldwide, especially oceanic islands, volcanic provinces, and ancient lava plateaus.
