igneous
Dark fine-grained rock, likely basalt
Basalt (primarily plagioclase feldspar and pyroxene, with possible olivine and iron oxides)
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, massive, and dull to slightly glassy; Mohs hardness about 5–6, with no obvious cleavage. The specimen appears weathered and granular, but the photo is insufficient for definitive identification.
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Physical properties
Typically dark gray to black, fine-grained, massive, and dull to slightly glassy; Mohs hardness about 5–6, with no obvious cleavage. The specimen appears weathered and granular, but the photo is insufficient for definitive identification.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and continental flood-basalt provinces. Individual specimens may range from modern to Precambrian in age.
Uses & applications
Crushed basalt is widely used as aggregate, road base, concrete material, and railroad ballast; attractive polished pieces are used decoratively, but this small fragment has little gem 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
Look for a uniformly fine texture, dark color, high density, and tiny feldspar crystals; a streak test, magnetism test, and fresh fracture would help distinguish it from slag or other dark rocks. Common worldwide, especially volcanic regions and seafloor-derived terranes.
