igneous
Likely basalt
Basalt; fine-grained mafic volcanic rock, typically rich in plagioclase, pyroxene, and olivine
AI-generated identification · may be incorrectDark gray to black, dense, and fine-grained with a dull to slightly vitreous luster; hardness about 5–6 Mohs, no obvious cleavage, and specific gravity roughly 2.8–3.0. The granular sparkle may reflect tiny crystals or a weathered surface.
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Physical properties
Dark gray to black, dense, and fine-grained with a dull to slightly vitreous luster; hardness about 5–6 Mohs, no obvious cleavage, and specific gravity roughly 2.8–3.0. The granular sparkle may reflect tiny crystals or a weathered surface.
Formation & geological history
Forms when basaltic lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and lava flows; specimens may range from recent to Precambrian in age. The photograph alone cannot establish a precise locality or age.
Uses & applications
Used extensively as crushed aggregate, road base, railway ballast, building stone, and occasionally as architectural facing or decorative cobbles. Ordinary hand specimens generally have minimal collector value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and commonly forms the ocean floor. It may be confused with gabbro, which has visibly larger crystals, or slag, which is often vesicular and unusually lightweight.
Field identification & locations
Identify it by its black, compact, heavy appearance and tiny interlocking crystals; a streak test is usually gray, and it commonly does not react with dilute acid. Common localities include Hawaii, Iceland, the Deccan Traps, oceanic islands, and volcanic regions worldwide.
