Rock Identifier
Amygdaloidal Basalt (Amygdaloidal Basalt (Host rock) with Quartz or Calcite amygdules) — igneous
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
Identified More igneous →

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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.

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