igneous
Amygdaloidal basalt (vesicular basalt)
Basalt, commonly containing amygdales of quartz, chalcedony, calcite, or zeolites
AI-generated identification · may be incorrectDark gray, fine-grained volcanic rock with abundant rounded vesicles filled by pale minerals; typically hardness 5–6, dull to faintly vitreous luster, and no visible cleavage. The light fillings are harder or softer depending on whether they are silica, calcite, or zeolite.
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Physical properties
Dark gray, fine-grained volcanic rock with abundant rounded vesicles filled by pale minerals; typically hardness 5–6, dull to faintly vitreous luster, and no visible cleavage. The light fillings are harder or softer depending on whether they are silica, calcite, or zeolite.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near the surface, trapping gas bubbles later filled by groundwater minerals. Its age cannot be determined from the image; such rocks range from recent eruptions to Archean examples.
Uses & applications
Used as aggregate, road stone, riprap, and occasionally as a decorative or lapidary material. Attractive amygdales may be cut or polished for specimens.
Geological facts
The filled vesicles are called amygdales, producing the distinctive “almond-shaped” pattern. Common fillings include agate, quartz, calcite, and stilbite.
Field identification & locations
Identify by its black-gray fine-grained matrix, rounded cavities, and contrasting mineral fillings; test suspect calcite fillings with dilute acid and quartz fillings with a steel file. Common in volcanic regions and flood-basalt provinces worldwide.
