igneous
Porphyritic basalt (likely amygdaloidal basalt)
Basalt containing larger plagioclase crystals and pale vesicle-fill minerals, commonly calcite, zeolite, or quartz
AI-generated identification · may be incorrectDark gray, fine-grained volcanic rock with conspicuous cream-colored phenocrysts or amygdules; typically Mohs 5–6, dull to subtly silky luster, interlocking crystalline texture, and no obvious cleavage. Exact identification is limited by the photograph.
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Physical properties
Dark gray, fine-grained volcanic rock with conspicuous cream-colored phenocrysts or amygdules; typically Mohs 5–6, dull to subtly silky luster, interlocking crystalline texture, and no obvious cleavage. Exact identification is limited by the photograph.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, leaving larger crystals in a finer matrix; vesicles may later have filled with secondary minerals. It may range from ancient volcanic terrain to geologically recent lava flows.
Uses & applications
Used as aggregate, road stone, landscaping material, and occasionally as a decorative or cabochon-quality specimen. Collector value is usually modest.
Geological facts
The pale spots could be plagioclase feldspar phenocrysts, calcite-filled vesicles, or weathered inclusions; a hardness, streak, acid, and close-grained inspection would distinguish them.
Field identification & locations
Common in volcanic regions worldwide, including Iceland, India, the western United States, and oceanic islands. Test whether spots are raised crystals or cavities, and use dilute hydrochloric acid to check for calcite fizz.
