igneous
Polished basalt pebble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark brown-black, rounded and likely water-worn or polished, with a dull to subtly vitreous luster and small lighter mineral grains. Hardness is approximately 5–6 Mohs; massive, non-cleavable, and typically dense (specific gravity about 2.8–3.0).
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Physical properties
Dark brown-black, rounded and likely water-worn or polished, with a dull to subtly vitreous luster and small lighter mineral grains. Hardness is approximately 5–6 Mohs; massive, non-cleavable, and typically dense (specific gravity about 2.8–3.0).
Formation & geological history
Forms when mafic lava rapidly cools at or near Earth’s surface, commonly from oceanic crust, volcanic islands, and continental flood-basalt provinces. Basalt ranges from Archean to modern age; this specimen’s age cannot be determined visually.
Uses & applications
Used as aggregate, road stone, riprap, paving, and occasionally as a decorative or lapidary stone. Tumbled basalt has modest collecting value.
Geological facts
The rounded shape indicates abrasion during transport, while the glossy surface may reflect polishing or natural smoothing. Basalt is the most abundant volcanic rock on Earth.
Field identification & locations
Identify by its dark, heavy, fine-grained appearance and lack of visible large crystals; a streak test is usually gray-black, and it is generally nonmagnetic or weakly magnetic. Common in volcanic regions and river gravels worldwide.
