Amygdaloidal Basalt

Physical Properties

Hardness: 5-6 (matrix); Color: Dark reddish-brown to black with white/light gray mineral inclusions (amygdules); Luster: Dull/earthy matrix with vitreous inclusions; Crystal structure: Aphanitic (fine-grained) with secondary mineral fillings; Specific gravity: 2.8-3.0.

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava. The holes (vesicles) created by gas bubbles were later filled with secondary minerals like quartz, calcite, or zeolites through hydrothermal precipitation, often during the Proterozoic era (approx. 1.1 billion years ago in regions like the Midcontinent Rift).

Uses & Applications

Used primarily as crushed stone for construction and road base. Lapidary artists sometimes polish decorative specimens, especially those containing colorful minerals like agate or epidote.

Geological Facts

The word 'amygdule' comes from the Latin word 'amygdala', meaning almond, referring to the almond-shaped mineral deposits within the rock. This specific appearance is very common in the Lake Superior region of North America.

Field Identification & Locations

Identify in the field by its dark, heavy basaltic matrix peppered with distinct, light-colored 'spots' or 'veins' that are actually mineral-filled gas bubbles. Common in volcanic terrains like the Keweenaw Peninsula, Oregon, and Hawaii.

Identified on: 4/25/2026

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