igneous
Tumbled basalt
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar, clinopyroxene, and commonly olivine or magnetite
AI-generated identification · may be incorrectDark gray to black with brown oxidation tones, opaque, dense, and weakly vitreous to dull; typically hardness 5–6 Mohs, fine-grained, with no visible cleavage. The smooth rounded shape and glossy surface indicate mechanical tumbling and polishing.
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Physical properties
Dark gray to black with brown oxidation tones, opaque, dense, and weakly vitreous to dull; typically hardness 5–6 Mohs, fine-grained, with no visible cleavage. The smooth rounded shape and glossy surface indicate mechanical tumbling and polishing.
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. Most examples formed during the Cenozoic or older volcanic episodes, but the specimen’s age cannot be determined visually.
Uses & applications
Used as aggregate, road stone, building stone, and occasionally in landscaping; polished pieces are sold as pocket stones, meditation stones, or decorative specimens.
Geological facts
Its high density and dark color reflect abundant ferromagnesian minerals and often magnetite. A magnet may show weak attraction if magnetite is present.
Field identification & locations
Identify it by its uniformly dark, fine-grained texture, high density, and lack of obvious crystals or layering; unlike glass, it usually feels stony and may show tiny mineral grains under magnification. Common sources include Hawaii, Iceland, India, and oceanic volcanic regions.
