igneous
Amygdaloidal Basalt
Extrusive Igneous Rock with Secondary Mineral Fillings
Hardness: 6-7 on Mohs scale for the quartz/silica amygdules, 5-6 for the basalt matrix; Color: Dark grey to greenish-black matrix with lighter green, white, or pinkish amygdule spots; Luster: Dull to sub-vitreous;…
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale for the quartz/silica amygdules, 5-6 for the basalt matrix; Color: Dark grey to greenish-black matrix with lighter green, white, or pinkish amygdule spots; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained) groundmass with microcrystalline or amorphous secondary mineral infillings.
Formation & geological history
Formed from cooling volcanic lava flows where gas bubbles (vesicles) were later filled with secondary minerals such as quartz, calcite, or zeolites circulating in hydrothermal fluids.
Uses & applications
Used primarily in crushed stone for construction, aggregate, roadbeds, and as a popular specimen for rock collectors.
Geological facts
The characteristic rounded spots are called amygdules, which are former gas cavities in lava that became filled with minerals long after the rock originally solidified.
Field identification & locations
Found easily in ancient volcanic regions and glacial drift gravels, identifiable by its contrasting dark fine-grained background dotted with lighter mineral pockets.
