Amygdaloidal Basalt

Physical Properties

Hardness: 5-7 (groundmass) with softer fillings; Color: Dark grey to reddish-brown matrix with white inclusions; Luster: Dull/Earthly matrix, vitreous/pearly inclusions; Texture: Vesicular/Porphyritic; Gravity: 2.8-3.0.

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava. The white 'spots' (amygdules) 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

Used primarily as crushed stone for construction, road base, and railroad ballast. Decorative specimens are popular among amateur collectors and educators.

Geological Facts

The word 'amygdule' comes from the Latin 'amygdala', meaning almond, referring to the almond-shaped mineral deposits. This rock is a common host for semi-precious gemstones like agates and copper in regions like Lake Superior.

Field Identification & Locations

Identify by looking for a dark, dense, fine-grained volcanic matrix with distinct, rounded light-colored mineral 'pockets'. Commonly found in volcanic fields, oceanic islands, and ancient rift zones like the Keweenaw Peninsula.

Identified on: 5/14/2026

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