igneous
Likely basalt or fine-grained volcanic rock
Basalt; chiefly plagioclase feldspar, pyroxene, and minor olivine/iron oxides
AI-generated identification · may be incorrectDark gray, fine-grained, dull to weakly vitreous, with scattered rusty-orange weathering spots; typical hardness about 6, no obvious cleavage, and specific gravity roughly 2.8–3.1. The visible flat faces may be natural fracture surfaces or saw-cut surfaces.
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Physical properties
Dark gray, fine-grained, dull to weakly vitreous, with scattered rusty-orange weathering spots; typical hardness about 6, no obvious cleavage, and specific gravity roughly 2.8–3.1. The visible flat faces may be natural fracture surfaces or saw-cut surfaces.
Formation & geological history
Basalt forms when low-silica lava cools rapidly at or near Earth’s surface, commonly in volcanic fields and oceanic crust. Individual pieces may range from recent eruptions to ancient Precambrian flows, but age cannot be determined from the photograph.
Uses & applications
Used extensively as crushed aggregate, road stone, railroad ballast, concrete aggregate, paving, and dimension stone; attractive polished pieces may be collected, though this specimen appears ordinary.
Geological facts
Rust-colored patches likely represent oxidation of iron-bearing minerals. Basalt can be distinguished from similarly dark rocks by its very fine texture and lack of visible quartz or large crystals.
Field identification & locations
Common worldwide, especially in Hawaii, Iceland, India’s Deccan Traps, oceanic islands, and continental volcanic provinces. A hand lens, streak test, magnet test, and examination of fresh surfaces would improve identification.
