igneous
Vesicular basalt (likely amygdaloidal basalt)
Basalt, a mafic volcanic rock composed mainly of plagioclase feldspar, clinopyroxene, and commonly olivine; vesicles are partly mineral-filled amygdules.
AI-generated identification · may be incorrectDark gray, fine-grained matrix with abundant rounded gas cavities and pale mineral fillings; typically dull to weakly vitreous, hardness about 5–6, dense, with no obvious cleavage. The light spots may be calcite, zeolite, quartz, or feldspar.
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Physical properties
Dark gray, fine-grained matrix with abundant rounded gas cavities and pale mineral fillings; typically dull to weakly vitreous, hardness about 5–6, dense, with no obvious cleavage. The light spots may be calcite, zeolite, quartz, or feldspar.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, trapping gas bubbles; cavities were later filled by groundwater minerals. It may range from recent lava to rocks hundreds of millions of years old, depending on locality.
Uses & applications
Used as aggregate, road base, riprap, landscaping stone, and occasionally as a decorative or lapidary material; usually has little gem value.
Geological facts
The holes are vesicles; mineral-filled holes are called amygdules. Its dark color and volcanic texture distinguish it from most sedimentary rocks.
Field identification & locations
Identify by its dark fine-grained groundmass, irregular holes, and heavy weight; a knife generally does not scratch fresh basalt. Common in volcanic regions such as Hawaii, Iceland, the Pacific Northwest, and oceanic crust.
