igneous
Dark gray mafic rock, likely basalt
Basalt; chiefly plagioclase feldspar and pyroxene, commonly with minor olivine and iron oxides
AI-generated identification · may be incorrectFine-grained, dark gray to black, generally dull to subvitreous luster; Mohs hardness about 5–6, high density, and little or no visible cleavage. The specimen appears weathered and may contain small vesicles or fracture cavities.
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Physical properties
Fine-grained, dark gray to black, generally dull to subvitreous luster; Mohs hardness about 5–6, high density, and little or no visible cleavage. The specimen appears weathered and may contain small vesicles or fracture cavities.
Formation & geological history
Basalt forms when low-silica lava cools rapidly at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and flood-basalt provinces. Its exact age cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, riprap, and building stone; attractive polished basalt is also used decoratively. Ordinary hand specimens have limited commercial value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. It commonly develops a rusty weathering rind from oxidation of iron-bearing minerals.
Field identification & locations
Identify it by its dark color, fine texture, heavy feel, and absence of obvious individual crystals; a streak test is usually gray, and it is not strongly reactive with dilute hydrochloric acid. Common worldwide, especially volcanic regions and former lava fields.
