igneous
Polished black basalt
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine and iron-titanium oxides
AI-generated identification · may be incorrectDark gray to black, fine-grained, massive, and weakly vitreous to dull; typically Mohs 5–6, with no visible cleavage and a specific gravity around 2.8–3.0. The glossy surface appears polished or naturally weathered, so texture and vesicles may be obscured.
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Physical properties
Dark gray to black, fine-grained, massive, and weakly vitreous to dull; typically Mohs 5–6, with no visible cleavage and a specific gravity around 2.8–3.0. The glossy surface appears polished or naturally weathered, so texture and vesicles may be obscured.
Formation & geological history
Basalt forms when low-silica, iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, shield volcanoes, and flood-basalt provinces. Most examples range from recent volcanic material to Precambrian specimens, but age cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road stone, railway ballast, riprap, building stone, and polished decorative material; attractive pieces are sold as curios or lapidary specimens.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant mafic minerals and opaque iron oxides.
Field identification & locations
Identify by its uniformly fine-grained black appearance, high density, and lack of layering; a streak, magnetism test, and fresh fracture help distinguish it from gabbro, slate, or slag. Common worldwide, especially volcanic regions and oceanic islands.
