igneous
Polished basalt (vesicular basalt pebble)
Basalt, primarily plagioclase feldspar and pyroxene, commonly with olivine and iron oxides
AI-generated identification · may be incorrectDark gray to black, fine-grained, opaque, and typically vitreous to dull; Mohs hardness about 6, with no prominent cleavage and specific gravity around 2.8–3.0. The cavities are vesicles, some later enlarged or weathered, and the glossy surface is polished or naturally water-worn.
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Physical properties
Dark gray to black, fine-grained, opaque, and typically vitreous to dull; Mohs hardness about 6, with no prominent cleavage and specific gravity around 2.8–3.0. The cavities are vesicles, some later enlarged or weathered, and the glossy surface is polished or naturally water-worn.
Formation & geological history
An extrusive igneous rock formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and hotspot regions. Basalt ranges from ancient Precambrian examples to actively forming modern lava.
Uses & applications
Used as aggregate, road stone, paving, ballast, and building stone; polished pieces are sold as decorative pebbles, massage stones, or inexpensive collectibles. Typical value is about $3–$20 for a small polished specimen, or up to $30–$75 for an unusually attractive piece.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Vesicles formed from gas bubbles trapped in cooling lava; dark fillings may be iron oxides or secondary minerals.
Field identification & locations
Identify it by its black, dense, fine-grained appearance, rounded polish, and occasional gas cavities; it should not show abundant visible quartz or large crystals. Common sources include Hawaii, Iceland, India’s Deccan Traps, Mexico, the western United States, and oceanic volcanic islands.
