igneous
Tumbled basalt
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, and subvitreous to dull; hardness about 5–6, no visible cleavage, and specific gravity roughly 2.8–3.1. The specimen appears water-worn or polished, with subtle brown metallic-looking mineral grains.
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Physical properties
Typically dark gray to black, fine-grained, and subvitreous to dull; hardness about 5–6, no visible cleavage, and specific gravity roughly 2.8–3.1. The specimen appears water-worn or polished, with subtle brown metallic-looking mineral grains.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and flood-basalt provinces. Individual pieces may be geologically millions to billions of years old, depending on source.
Uses & applications
Used as crushed aggregate, road stone, railway ballast, riprap, and building stone; polished pieces are sold as inexpensive decorative or lapidary specimens.
Geological facts
Its dark color reflects abundant ferromagnesian minerals, while tiny crystals indicate relatively rapid cooling. The rounded shape and glossy surface suggest natural river/beach abrasion or tumbling rather than a crystal habit.
Field identification & locations
Identify it by its uniformly dark, dense, fine-grained texture and lack of obvious layering or large crystals; a streak test is usually gray to black. Common worldwide, especially near volcanic terrains and ocean beaches.
