igneous
Iron-rich basalt or weathered ironstone
Basalt, likely variolitic/vesicular and coated with iron oxides; composition mainly plagioclase, pyroxene, and magnetite/hematite
AI-generated identification · may be incorrectDark gray to black, locally rusty brown, with dull to weakly vitreous luster and possible vesicles; typical hardness is about 5–6 Mohs, with no obvious cleavage and fine-grained crystalline texture. The glossy areas may be water, polish, or a secondary mineral coating.
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Physical properties
Dark gray to black, locally rusty brown, with dull to weakly vitreous luster and possible vesicles; typical hardness is about 5–6 Mohs, with no obvious cleavage and fine-grained crystalline texture. The glossy areas may be water, polish, or a secondary mineral coating.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly in volcanic flows and oceanic crust; it may be tens of thousands to billions of years old. Brown coloration reflects weathering and oxidation of iron-bearing minerals.
Uses & applications
Used as crushed aggregate, road stone, riprap, building stone, and occasionally as a decorative material; specimens have modest collecting value unless unusually textured or mineralized.
Geological facts
Basalt is the most widespread volcanic rock on Earth and forms much of the ocean floor. Strong attraction to a magnet would indicate abundant magnetite, while a rusty streak suggests iron-oxide weathering.
Field identification & locations
Identify it by its dense dark color, fine grain, angular fracture, and occasional magnetic response; distinguish it from slag by its natural vesicles and crystalline texture. Common in volcanic regions, oceanic islands, and ancient lava provinces.
