Basalt with vesicles (Amygdaloidal Basalt)

Physical Properties

Hardness: 5-6 (Mohs scale); Color: Dark grey to brown with tan or white secondary mineral inclusions; Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained) matrix; Cleavage: None/Irregular;

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava. The holes (vesicles) were created by gas bubbles during eruption, which were later filled with secondary minerals like quartz, calcite, or zeolites through groundwater infiltration.

Uses & Applications

Used primarily in construction aggregate, road base, and as decorative river stones or for lapidary tumbling.

Geological Facts

Basalt is the most common rock in Earth's crust and makes up most of the ocean floor. When the gas bubbles in basalt are filled with minerals, they are called 'amygdules'—Greek for 'almonds'.

Field Identification & Locations

Identify in the field by its high density, dark fine-grained matrix, and the presence of small, rounded, light-colored mineral spots. Commonly found in volcanic regions and glacial till.

Identified on: 4/15/2026

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