igneous
Basalt
Basalt; primarily plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray-brown, fine-grained and massive, with dull to slightly glassy luster; typical hardness is about 5–6 Mohs and specific gravity 2.8–3.0. Small pale or dark crystals may be visible, but the specimen appears weathered and may be vesicle-poor.
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Physical properties
Dark gray-brown, fine-grained and massive, with dull to slightly glassy luster; typical hardness is about 5–6 Mohs and specific gravity 2.8–3.0. Small pale or dark crystals may be visible, but the specimen appears weathered and may be vesicle-poor.
Formation & geological history
An extrusive mafic igneous rock formed when low-silica basaltic lava cooled rapidly at or near Earth’s surface. Most examples are Cenozoic or younger, though basalt occurs throughout geologic time, including oceanic crust and ancient volcanic provinces.
Uses & applications
Widely used as crushed aggregate, road base, railway ballast, concrete aggregate, and dimension stone; attractive polished pieces are used decoratively but are not usually gemstone material.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify it by its dark color, heavy feel, fine texture, and lack of obvious light quartz grains; a streak test is usually gray to dark gray. Common localities include Hawaii, Iceland, the Columbia River Basalt Province, India’s Deccan Traps, and oceanic islands.
