igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (Mafic extrusive volcanic rock with secondary mineral infilling)
Hardness: 5-6 (basalt matrix); Color: Dark grey, reddish-brown, or black with white, pink, or green vesicles; Luster: Dull to earthy; Crystal structure: Aphanitic (fine-grained) matrix; Cleavage: None/Irregular fracture.
- Hardness
- 5-6 (basalt matrix)
- Luster
- Dull to earthy
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Physical properties
Hardness: 5-6 (basalt matrix); Color: Dark grey, reddish-brown, or black with white, pink, or green vesicles; Luster: Dull to earthy; Crystal structure: Aphanitic (fine-grained) matrix; Cleavage: None/Irregular fracture.
Formation & geological history
Formed from the rapid cooling of basaltic lava. During cooling, gas bubbles (vesicles) are trapped in the rock. Later, mineral-rich groundwater flows through these voids, depositing minerals like quartz, calcite, zeolites, or epidote to create the 'amygdules' (the light-colored spots).
Uses & applications
Used primarily as crushed stone for road construction and concrete aggregate. Attractive specimens are popular for amateur geology collections and educational purposes.
Geological facts
The word 'amygdaloid' comes from 'amygdala,' the Latin word for almond, because the mineral-filled pits often resemble almonds. This rock type is common in the Lake Superior region of North America and contains the world's largest deposits of native copper.
Field identification & locations
Identify in the field by looking for a heavy, dark, fine-grained rock with round or oval 'polka dots' of a different mineral. Can be found in volcanic regions like the Pacific Northwest, Lake Superior, or Iceland.
