igneous
Rounded basaltic river pebble
Basalt, predominantly plagioclase and pyroxene, commonly with olivine and iron oxides
AI-generated identification · may be incorrectDark brown-gray, fine-grained, dull to slightly vitreous luster, massive with no visible cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.1. Its smooth, elongated shape indicates prolonged water abrasion and weathering.
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Physical properties
Dark brown-gray, fine-grained, dull to slightly vitreous luster, massive with no visible cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.1. Its smooth, elongated shape indicates prolonged water abrasion and weathering.
Formation & geological history
Basalt forms when low-silica, iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, generally in volcanic regions and oceanic crust. The pebble is likely geologically old volcanic rock subsequently transported in a stream or beach environment.
Uses & applications
Used as aggregate, road metal, riprap, landscaping stone, and occasionally for polished decorative objects; this unremarkable pebble has little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and makes up much of the ocean floor. Brown surface coloration commonly reflects weathering and oxidation of iron-bearing minerals.
Field identification & locations
Identify it by its heavy, fine-grained dark appearance, lack of obvious layers, and possible attraction to a magnet if magnetite-rich; a streak, density, and thin-section test would confirm it. Common near volcanic terrains, river gravels, and beaches.
