igneous
Vesicular basalt with an amygdule cavity
Vesicular basalt, likely with secondary quartz or calcite mineralization
AI-generated identification · may be incorrectDark gray, fine-grained volcanic rock with irregular gas vesicles and a larger cavity lined by pale cream crystalline material. Basalt typically has Mohs hardness 5–6, dull to slightly vitreous luster, no visible cleavage, and specific gravity about 2.8–3.0.
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Physical properties
Dark gray, fine-grained volcanic rock with irregular gas vesicles and a larger cavity lined by pale cream crystalline material. Basalt typically has Mohs hardness 5–6, dull to slightly vitreous luster, no visible cleavage, and specific gravity about 2.8–3.0.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, trapping gas bubbles; later groundwater deposited minerals in the cavities. Such basalt is generally Cenozoic to modern, though similar material occurs in older volcanic provinces.
Uses & applications
Used mainly as landscaping stone, aggregate, ballast, and specimen material; attractive mineral-lined cavities may have modest collector interest.
Geological facts
The cavity is an amygdule or vug, and its pale lining may be calcite, quartz, zeolite, or weathered alteration minerals; a photo alone cannot determine which confidently.
Field identification & locations
Common in volcanic regions, including Iceland, the Pacific Northwest, India, and Mexico. Identify basalt by its dark fine-grained matrix, vesicles, and lack of foliation; acid testing or hardness/streak tests can help distinguish calcite from quartz.
