igneous
Amygdaloidal Basalt
Vesicular Basalt with Amygdules
Hardness of 5-6 on the Mohs scale, reddish-brown weathered surface due to iron oxidation with lighter mineral infillings (amygdules) of zeolites or quartz, dull to sub-vitreous luster, fine-grained aphanitic groundmass with distinct rounded amygdules, specific gravity around 2.7-3.0.
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Physical properties
Hardness of 5-6 on the Mohs scale, reddish-brown weathered surface due to iron oxidation with lighter mineral infillings (amygdules) of zeolites or quartz, dull to sub-vitreous luster, fine-grained aphanitic groundmass with distinct rounded amygdules, specific gravity around 2.7-3.0.
Formation & geological history
Formed from mafic volcanic lava flows where escaping gas bubbles create vesicles. Over time, mineral-rich groundwater percolates through these cavities, precipitating secondary minerals to fill them, creating amygdules.
Uses & applications
Used primarily as crushed stone in construction, railroad ballast, and occasionally as decorative landscaping rock or tumbled stones by hobbyists.
Geological facts
The reddish-brown color is typically caused by the oxidation of iron-rich minerals like olivine and pyroxene during weathering. When amygdules consist of agate or quartz, they can sometimes be polished into ornamental cabochons.
Field identification & locations
Easily identified by its dark, fine-grained volcanic matrix peppered with distinct round or oval lighter-colored mineral pockets. Found worldwide in ancient and modern volcanic regions, particularly around lake superiors and basaltic provinces.
