igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (Host rock) with Quartz or Calcite amygdules
Hardness: 6-7 (basalt), 3 (if calcite inserts) to 7 (if quartz). Color: Dark grey to reddish-brown matrix with white circular inclusions. Luster: Dull or earthy matrix with vitreous inclusions. Structure: Vesicular texture filled by secondary minerals.
- Hardness
- 6-7 (basalt), 3 (if calcite inserts) to 7 (if quartz)
- Luster
- Dull or earthy matrix with vitreous inclusions
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Physical properties
Hardness: 6-7 (basalt), 3 (if calcite inserts) to 7 (if quartz). Color: Dark grey to reddish-brown matrix with white circular inclusions. Luster: Dull or earthy matrix with vitreous inclusions. Structure: Vesicular texture filled by secondary minerals.
Formation & geological history
Formed from the rapid cooling of basaltic lava. Gas bubbles (vesicles) trapped during solidification were later filled with secondary minerals precipitated from groundwater millions of years later. Age varies, often associated with Precambrian or Cenozoic volcanic activity.
Uses & applications
Used primarily as road base or aggregate in construction. High-quality specimens with colorful agate or zeolite fillings are sought after by rockhounds and collectors for display.
Geological facts
The word 'amygdule' is derived from the Greek word 'amygdala', meaning almond, referring to the almond-shaped holes in the rock. Famous examples include the Lake Superior Agates found in similar basaltic hosts.
Field identification & locations
Identify in the field by the dark, heavy, fine-grained volcanic matrix containing distinct, rounded, white or translucent 'spots'. Commonly found in volcanic regions such as the Pacific Northwest, Lake Superior region, and India.
