Amygdaloidal Basalt

Physical Properties

Hardness: 5-6 (Mohs scale), Color: Dark grey to black matrix with lighter tan or white inclusions, Luster: Dull/earthy matrix with pearly to vitreous inclusions, Crystal structure: Aphanitic (fine-grained) matrix with mineral-filled vesicles, Specific gravity: 2.8-3.0.

Formation & Geological History

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

Uses & Applications

Commonly used in construction as crushed stone or aggregate. Decorative specimens are popular among amateur geologists and collectors for their unique 'polka-dot' appearance.

Geological Facts

The word 'amygdule' comes from the Latin 'amygdala', meaning almond, referring to the almond-shaped mineral fillings. This specific type of rock is famous in the Lake Superior region for sometimes containing native copper in the air pockets.

Field Identification & Locations

Identify by looking for a dense, dark volcanic matrix containing rounded or elliptical spots of a different color. Common in volcanic terrains such as the Pacific Northwest, Iceland, and the Keweenaw Peninsula.

Identified on: 4/26/2026

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