igneous
Dark fine-grained volcanic rock, likely basalt
Basalt; mainly plagioclase feldspar and pyroxene, commonly with minor olivine and magnetite
AI-generated identification · may be incorrectTypically 5–6 on the Mohs scale, dark gray to black, fine-grained and massive, with a dull to faintly vitreous luster. It commonly shows conchoidal-to-uneven fracture and no visible cleavage; density is about 2.8–3.0 g/cm³.
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Physical properties
Typically 5–6 on the Mohs scale, dark gray to black, fine-grained and massive, with a dull to faintly vitreous luster. It commonly shows conchoidal-to-uneven fracture and no visible cleavage; density is about 2.8–3.0 g/cm³.
Formation & geological history
Basalt forms when low-silica mafic lava rapidly cools at or near Earth’s surface, producing tiny crystals; it may range from ancient oceanic crust to geologically recent lava flows. The specimen’s sharp break and uniform texture are consistent with basalt, though the photo cannot confirm it definitively.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, riprap, and building stone; attractive polished varieties are used decoratively. Common specimens generally have little jewelry or collector value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the oceanic crust. Vesicles, if present, may later be filled by minerals such as zeolites, calcite, or quartz.
Field identification & locations
Look for a heavy, dark, fine-grained rock that does not fizz with dilute hydrochloric acid and may contain tiny pale feldspar grains or magnetic magnetite. Common worldwide, especially in volcanic regions and oceanic crust exposures.
