igneous
Tumbled black basalt
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, with possible olivine and magnetite
AI-generated identification · may be incorrectTypically Mohs 5–6; black to dark gray, dense, opaque, and dull to subtly vitreous when polished. Fine-grained crystalline texture; no visible cleavage, commonly weakly magnetic if magnetite-rich.
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Physical properties
Typically Mohs 5–6; black to dark gray, dense, opaque, and dull to subtly vitreous when polished. Fine-grained crystalline texture; no visible cleavage, commonly weakly magnetic if magnetite-rich.
Formation & geological history
Forms when basaltic lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, lava flows, and volcanic islands. Individual specimens may range from recent volcanic material to billions of years old.
Uses & applications
Used as aggregate, road stone, ballast, and occasionally building stone; polished pieces are sold as inexpensive decorative stones and lapidary material.
Geological facts
Its dark color reflects iron- and magnesium-rich minerals. A smooth glossy surface usually indicates human tumbling or polishing rather than natural weathering.
Field identification & locations
Identify by its very high density, uniform dark fine-grained appearance, and lack of distinct crystals; streak is usually gray, and some pieces attract a magnet. Common worldwide in volcanic regions.
