igneous
Tumbled gabbro or basalt
Dark mafic igneous rock, likely gabbro (plutonic) or basalt (volcanic), composed mainly of plagioclase and pyroxene
AI-generated identification · may be incorrectHardness about 6–7 Mohs; dark gray to black, mottled white-gray, with a dull to weakly vitreous luster and fine-to-medium crystalline texture. The rounded, glossy surface indicates tumbling and polishing; individual crystals are poorly resolved.
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Physical properties
Hardness about 6–7 Mohs; dark gray to black, mottled white-gray, with a dull to weakly vitreous luster and fine-to-medium crystalline texture. The rounded, glossy surface indicates tumbling and polishing; individual crystals are poorly resolved.
Formation & geological history
Formed from cooling iron- and magnesium-rich magma, either slowly underground as gabbro or rapidly at the surface as basalt; age cannot be determined from the photograph. Common sources include oceanic crust and ancient mafic intrusions.
Uses & applications
Used as aggregate, road stone, building stone, and decorative material; polished pieces are sold as worry stones or inexpensive lapidary specimens. Typical value is about $2–$15 for a small tumbled piece, or $10–$40 if marketed as a distinctive polished specimen.
Geological facts
Its dark color reflects abundant mafic minerals, while the pale patches are likely plagioclase feldspar or alteration minerals. A precise identification requires a fresh surface, texture examination, streak, and possibly thin-section or chemical testing.
Field identification & locations
Commonly found in volcanic terrains, oceanic crust, and mafic intrusive bodies. It is distinguished from obsidian by its visible crystalline/mottled texture and from slag by its natural mineral grains and lack of abundant bubbles.
