Amygdaloidal Basalt

Physical Properties

Hardness: 5-6 (Mohs scale), Color: Dark grey to black matrix with lighter cream or green fillings. Luster: Dull/Earthy. Crystal structure: Aphanitic (fine-grained) with secondary mineral fillings (amygdules). Cleavage: None.

Formation & Geological History

Formed from the cooling of gas-rich lava flows. As the lava cools, gas bubbles are trapped, creating voids (vesicles). Over millions of years, mineral-rich groundwater flows through, depositing minerals like quartz, calcite, or zeolites into the holes.

Uses & Applications

Used primarily in road construction (aggregate), as decorative landscaping stones, and occasionally as semi-precious cabinet specimens if the amygdules contain colorful minerals like agate or turquoise.

Geological Facts

The name comes from the Greek word 'amygdale' meaning almond, referring to the almond-shaped mineral deposits. Many of the famous Lake Superior agates formed inside the vesicles of this type of basalt.

Field Identification & Locations

Identify in the field by looking for dark, heavy volcanic rocks with lighter-colored 'dots' or 'almond' shapes on the surface. Commonly found in volcanic regions such as the Pacific Northwest or the Great Lakes 'Keweenawan' rift zones.

Identified on: 4/16/2026

Mode: Standard