igneous
Tumbled basalt pebble
Basalt (fine-grained mafic volcanic rock; commonly plagioclase, pyroxene, and iron-titanium oxides)
AI-generated identification · may be incorrectDark gray-black with rusty yellow-brown weathering or iron-oxide staining; fine-grained, massive texture, dull to faintly vitreous luster, and no visible cleavage. Hardness is roughly 5–6 Mohs; specific gravity about 2.8–3.0.
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Physical properties
Dark gray-black with rusty yellow-brown weathering or iron-oxide staining; fine-grained, massive texture, dull to faintly vitreous luster, and no visible cleavage. Hardness is roughly 5–6 Mohs; specific gravity about 2.8–3.0.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, ranging from ancient Precambrian flows to modern volcanic eruptions. Its rounded shape indicates abrasion in a stream, beach, or tumbling process.
Uses & applications
Used as crushed aggregate, road stone, riprap, railway ballast, and landscaping material; attractive polished pieces are inexpensive collectibles.
Geological facts
The ochre patches likely represent oxidation of iron-bearing minerals during weathering, not necessarily a separate mineral species.
Field identification & locations
Identify it by its heavy feel, dark fine-grained appearance, lack of large crystals, and inability to scratch glass consistently; common worldwide in volcanic terrains and river gravels.
