igneous
Dark basalt (likely vesicular basalt)
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine (approx. SiO₂ 45–52%)
AI-generated identification · may be incorrectFine-grained, dark gray to brown, dull to subtly vitreous; hardness about 5–6 Mohs, massive with no obvious cleavage. The surface appears weathered and may contain tiny vesicles or pale feldspar crystals.
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Physical properties
Fine-grained, dark gray to brown, dull to subtly vitreous; hardness about 5–6 Mohs, massive with no obvious cleavage. The surface appears weathered and may contain tiny vesicles or pale feldspar crystals.
Formation & geological history
An extrusive igneous rock formed when mafic lava rapidly cooled at or near Earth’s surface, commonly in volcanic flows, mid-ocean ridges, and hotspots. The specimen’s exact geological age cannot be determined from the image.
Uses & applications
Widely used as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive polished basalt is used decoratively. Ordinary hand specimens generally have little commercial collector value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. It is typically denser and darker than andesite or rhyolite.
Field identification & locations
Identify it by its dark color, fine texture, heavy feel, and possible tiny vesicles; a streak test is usually gray to black, while acid produces no reaction. Common localities include Hawaii, Iceland, the Columbia River Basalt, India’s Deccan Traps, and oceanic crust.
