igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (Mafic volcanic rock)
Hardness: 5-6 (Mohs scale); Color: Dark grey to purple-black matrix with white or light-colored circular infillings; Luster: Dull (matrix) to vitreous (fillings); Crystal structure: Aphanitic (fine-grained) with secondary mineral amygdules; Specific gravity: 2.8–3.0
- Hardness
- 5-6 (Mohs scale)
- Luster
- Dull (matrix) to vitreous (fillings)
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Dark grey to purple-black matrix with white or light-colored circular infillings; Luster: Dull (matrix) to vitreous (fillings); Crystal structure: Aphanitic (fine-grained) with secondary mineral amygdules; Specific gravity: 2.8–3.0
Formation & geological history
Formed from the rapid cooling of basaltic lava. Gas bubbles (vesicles) trapped in the cooling lava are later filled with secondary minerals like calcite, quartz, or zeolites through hydrothermal precipitation or groundwater percolation.
Uses & applications
Used primarily in geological educational collections, as aggregate in construction when found in large volumes, and sometimes as a decorative landscape stone.
Geological facts
The name comes from the Greek word 'amygdale' (almond), referring to the almond-shaped mineral deposits within the rock. This specific appearance is common in the Lake Superior region and ancient volcanic flows.
Field identification & locations
Identify in the field by looking for a dark, dense volcanic matrix containing small, rounded, distinct white or green 'dots'. Commonly found in areas with historic volcanic activity, such as the Keweenaw Peninsula in Michigan or the Deccan Traps.
