igneous
Vesicular basalt (likely weathered volcanic rock)
Basalt; primarily plagioclase feldspar, pyroxene, and sometimes olivine
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, with rounded gas vesicles; hardness about 5–6 Mohs, dull to slightly vitreous luster, and no prominent cleavage. This specimen appears unusually tan-brown from weathering or alteration.
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Physical properties
Typically dark gray to black, fine-grained, with rounded gas vesicles; hardness about 5–6 Mohs, dull to slightly vitreous luster, and no prominent cleavage. This specimen appears unusually tan-brown from weathering or alteration.
Formation & geological history
Extrusive mafic lava that cooled rapidly at or near Earth’s surface, commonly during volcanic eruptions; individual formation age cannot be determined from the image. Weathering can fill vesicles with secondary minerals and produce the brown coloration.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and building stone; attractive vesicular pieces may be collected or used decoratively, but this example has little gem value.
Geological facts
Vesicles are bubbles trapped as gas-rich lava solidified. If the cavities are later filled with minerals such as zeolite, calcite, or quartz, the rock is termed amygdaloidal basalt.
Field identification & locations
Identify by its fine texture, relatively high density, irregular vesicles, and lack of visible large crystals; common in volcanic regions worldwide, including Hawaii, Iceland, India, and the Pacific Northwest. A fresh broken surface is usually much darker than the weathered exterior.
