igneous
Vesicular basalt pebble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark blue-gray to gray, dull to faintly waxy luster, fine-grained and massive, with abundant rounded gas vesicles; hardness about 5–6 Mohs, no visible cleavage, specific gravity roughly 2.8–3.0.
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Physical properties
Dark blue-gray to gray, dull to faintly waxy luster, fine-grained and massive, with abundant rounded gas vesicles; hardness about 5–6 Mohs, no visible cleavage, specific gravity roughly 2.8–3.0.
Formation & geological history
Formed when fluid basaltic lava rapidly cooled at or near Earth’s surface, leaving trapped gas bubbles; its exact age cannot be determined from the photograph and may range from recent volcanic deposits to ancient oceanic crust.
Uses & applications
Commonly used as crushed aggregate, road stone, riprap, railway ballast, and landscaping material; attractive polished pieces may be sold as inexpensive decor or geological specimens.
Geological facts
The holes are vesicles, distinguishing vesicular basalt from many dense dark rocks; mineral-rich groundwater can later fill them with zeolites, calcite, or silica, producing amygdaloidal basalt.
Field identification & locations
Look for a heavy, fine-grained dark rock with tiny cavities and occasional pale feldspar crystals; common in volcanic regions, oceanic islands, flood-basalt provinces, and stream gravels. A streak, magnetism test, and fresh surface would improve identification.
