igneous
Basalt cobble in concrete aggregate
Basalt; chiefly plagioclase feldspar and pyroxene, with possible olivine and iron-titanium oxides
AI-generated identification · may be incorrectDark gray-brown, fine-grained and dense, with dull to weakly vitreous luster; hardness about 5–6 Mohs, no visible cleavage, and typical specific gravity of 2.8–3.0. The surrounding material appears concrete, but the central object is a natural basalt fragment.
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Physical properties
Dark gray-brown, fine-grained and dense, with dull to weakly vitreous luster; hardness about 5–6 Mohs, no visible cleavage, and typical specific gravity of 2.8–3.0. The surrounding material appears concrete, but the central object is a natural basalt fragment.
Formation & geological history
Basalt forms when low-silica magma rapidly cools at or near Earth’s surface, commonly in lava flows and oceanic crust; most examples range from recent to hundreds of millions of years old. The specimen was likely quarried and crushed for construction aggregate.
Uses & applications
Used extensively for road base, concrete aggregate, riprap, railroad ballast, paving, and occasionally building stone or landscaping. Ordinary aggregate pieces have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and commonly forms dark, fine-grained layers in oceanic crust. Vesicles, tiny holes, or small olivine grains may occur, though this fragment appears relatively massive.
Field identification & locations
Identify it by its dark color, heavy feel, fine texture, and lack of large visible crystals; a streak test is usually gray, and it does not fizz with dilute acid. Common worldwide, especially near volcanic provinces and aggregate quarries.
