Amygdaloidal Basalt

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Dark grey to reddish-brown groundmass with green, white, or glassy secondary mineral inclusions; Luster: Dull/earthy matrix with vitreous inclusions; Structure: Vesicular/Amygdaloidal; Specific Gravity: 2.8-3.0.

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava. The 'amygdules' (the green/light spots) form when gas bubbles (vesicles) in the cooling lava are later filled with secondary minerals like quartz, calcite, chlorite, or zeolites through hydrothermal activity.

Uses & Applications

Used primarily as crushed stone for aggregate in construction, road base, and concrete. Amygdaloidal varieties are popular among mineral collectors, especially when vesicles contain semi-precious stones like agates or zeolites.

Geological Facts

The word 'amygdule' comes from the Greek word 'amygdala', meaning almond, referring to the almond-shaped mineral fillings. These rocks are primary sources for native copper in regions like the Keweenaw Peninsula of Michigan.

Field Identification & Locations

Identify in the field by looking for a dark, fine-grained volcanic matrix containing distinct, rounded 'pockets' filled with different-colored minerals. Commonly found in ancient volcanic rift zones and oceanic crust exposures.

Identified on: 4/18/2026

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