Amygdaloidal Basalt

Physical Properties

Hardness: 5-6 (matrix); Color: Dark grey to greenish-brown matrix with green 'bubbles'; Luster: Dull to vitreous; Structure: Vesicular/Amygdaloidal; Specific Gravity: 2.8–3.0

Formation & Geological History

Formed from the rapid cooling of basaltic lava. The holes (vesicles) were created by gas bubbles. Later, hydrothermal fluids filled these holes with secondary minerals like epidote, quartz, or zeolites.

Uses & Applications

Primarily of interest to geological collectors and as a study in volcanic processes. Basalt itself is used extensively in construction and road base.

Geological Facts

The green spherical inclusions are called 'amygdules', derived from the Latin word for 'almond'. They provide a record of the mineral-rich fluids that flowed through the rock long after it solidified.

Field Identification & Locations

Identify by looking for a fine-grained volcanic matrix containing rounded, mineral-filled cavities. Commonly found in the Lake Superior region (Keweenawan Rift) and other ancient volcanic fields.

Identified on: 4/19/2026

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